Biaxially oriented film

ABSTRACT

An object of the present invention is to provide a thin biaxially oriented film excellent in dimensional stability against humidity change, as well as a magnetic recording medium and a film capacitor using the same. The present invention provides a single layered or laminated biaxially oriented film comprising an aromatic polyester (a) and a polyolefin (b) having a melting point of from 230 to 280° C., wherein the ratio of the polyolefin (b) is from 5 to 30% based on the entire weight of the film, and the film thickness is from 1 to 10 μm.

This application is a divisional of U.S. application Ser. No.10/587,392, filed on May 11, 2007, now U.S. Pat. No. 8,067,105, which isa National Stage of International Application No. PCT/JP2005/001639,filed on Jan. 28, 2005, which claims priority to Japanese priorityapplication Nos. 2004-021045 and 2004-021049 filed on Jan. 29, 2004 andJapanese priority application Nos. 2004-146245, 2004-146247 and2004-146244 filed on May 17, 2004, which are hereby incorporated byreference.

TECHNICAL FIELD

The present invention concerns a thin-walled biaxially oriented filmexcellent in dimensional stability to humidity change. Morespecifically, it relates to a thin-walled biaxially oriented film alsohaving an excellent withstand voltage characteristic. Further, theinvention relates to a biaxially oriented film suitable to use as a basefilm for magnetic recording media and film capacitors.

BACKGROUND ART

Since polyester films have an excellent thermal characteristic and aphysical characteristic, they have been used in various application usessuch as magnetic recording media, capacitors, flexible substrates,optical members, food packages, decorative use.

By the way, in magnetic recording media, particularly, magneticrecording media for use in data storage, requirement for characteristicsto the base films has become severer along with increase in the capacityand the density of tapes. In magnetic recording media for data storageadopting a linear track system such as QIC, DLT and, further, highcapacity super DLT, and LTO, the track pitch is made extremely narrowfor attaining higher capacity of tapes. Therefore, they involve aproblem of causing track deviation to generate errors in a case wheredimensional changes occur in the direction of the tape width. Thedimensional changes include those due to changes of temperature andhumidity and due to aging shrinkage in the transverse direction which iscaused upon repetitive running in high temperature and high humiditystates under high tension. In a case where the dimensional change islarge, it brings about track deviation to generate errors uponelectromagnetic conversion. For the sake of convenience of explanation,the advancing direction when a film is formed continuously is referredto as a film-forming direction, a continuous film-forming direction, alongitudinal direction or an MD direction and, also, the directionwithin a plane perpendicular to the film-forming direction is sometimesreferred to as a transverse direction or a width direction.

For solving such dimensional changes, JP-A No. 5-212787 discloses abiaxially oriented polyethylene-2,6-naphtalene dicarboxylate film inwhich the Young's modulus in the longitudinal direction (EM), theYoung's modulus in the transverse direction (ET) and the ratio betweenboth of the Young's modulus (ET/EM) are defined each within apredetermined range, and the shrinkage in the longitudinal direction,the temperature expansion coefficient in the longitudinal direction(αt), and the humidity expansion coefficient in the longitudinaldirection (αh) are defined. Further, the pamphlet of WO No. 99/29488discloses a biaxially oriented polyester film in which a thermalexpansion coefficient αt (×10⁻⁶/° C.) in the transverse direction, ahumidity expansion coefficient αh in the transverse direction (×10⁻⁶/%RH) and shrinkage P in the transverse direction to a load when the loadis applied in the longitudinal direction (ppm/g) are defined each withina predetermined range. Furthermore, the pamphlet of WO No 00/76749discloses a biaxially oriented polyester film in which the dimensionalchange in the transverse direction when left under weight in thelongitudinal direction, thermal expansion coefficient αt (×10⁻⁶/° C.) inthe transverse direction, the humidity expansion coefficient αh in thetransverse direction (×10⁻⁶/% RH) and a shrinkage P in the transversedirection to a load when the load is applied in the longitudinaldirection (ppm/g) are defined each within a predetermined range.

However, the methods proposed in the publications are attained bydefining the stretching conditions and the subsequent heat settingtreatment conditions within the predetermined ranges. For example, whilethe aging shrinkage in the transverse direction upon applying a load inthe longitudinal direction can be improved by increasing the Young'smodulus in the longitudinal direction of the base film, on the otherhand, from a view point of the polymer characteristic and the filmforming property as the Young's modulus in the longitudinal directionincreases, the upper limit for the Young's modulus in the transversedirection decreases, which results in increase in the dimensionalchanges due to the temperature/humidity change, etc. and no drasticsolution has not yet been attained.

Further, capacitors are manufactured by a method of stacking athermoplastic resin film such as of polyethylene terephthalate orpolypropylene and a thin metal film such as an aluminum foil and windingor laminating them. In recent years, along with the demand forsize-reduction of electric or electronic circuits, size-reduction andmounting has been progressing also for film capacitors and further heatresistance has been required in addition to an electric characteristic.Further, in the application use for automobiles, the range of use isextended not only to the use in a driver's cab but also to the inside anengine room, and film capacitors suitable to dimensional stability atfurther higher temperature and higher humidity have been required inaddition to the electric characteristic.

In view of the above, with an aim of solving the heat resistance offilms for use in capacitors, JP-A No. 2000-173855 discloses a method ofusing a polyethylene-2,6-naphthalate film and, with an aim of improvingthe electric characteristic thereof, a method of controlling the stateof crystallization, intrinsic viscosity, etc. has been proposed.However, the method has a limit in the further improvement of theelectric characteristic since this is a polar polymer.

On the other hand, as a thermoplastic resin with an excellent electriccharacteristic, syndiotactic polystyrene polymers have been known.However, since the syndiotactic polystyrene polymers are more difficultto be formed into a film compared with polyester resins and since theobtained film tends to be torn as well, improvement for thehandleability during manufacture of capacitors has been demanded.

By the way, the pamphlet of WO No. 97/32223 proposes films containingsyndiotactic polystyrene and polyethylene-2,6-naphthalate. However, suchfilms are optical materials for controlling the optical characteristicsuch as reflectance or transmittance and they are substantiallymonoaxially oriented films.

Further, JP-A No. 08-176329 proposes a void-containing polyester film inwhich a syndiotactic polystyrene is blended as a void former with apolyester resin and it discloses that the less deformability of thesyndiotactic polystyrene at a stretching temperature gives an effect onthe development of voids. However, as the thickness of the filmdecreases, since the effects of voids on various kinds ofcharacteristics increase, there may be a possibility in the applicationuse requiring thin film thickness, that various kinds of characteristicsnecessary for the application uses, for example, the mechanicalcharacteristic such as Young's modulus and the withstand voltagecharacteristic are deteriorated.

Further, as a film formed by laminating a syndiotactic polystyrene and apolyester, JP-A No. 8-48008 describes a laminate film with the ratio ofthe syndiotactic polystyrene layer of 70% or more.

Further, JP-A No. 2000-326467 proposes a multi-layered laminate film inwhich a layer comprising polyethyelen-2,6-naphthalate and a layercomprising a syndiotactic polystyrene are alternately laminated by 11layers or more. However, such a film intends to selectively reflect alight at a predetermined wavelength by optical interference due to thedifference of refractive index between layers.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide a thin-walled biaxiallyoriented film excellent in the dimensional stability to humidity change.

A further object of the invention is to provide a thin-walled biaxiallyoriented film having an excellent withstand voltage characteristic aswell.

A still further object of the invention is to provide a biaxiallyoriented film suitable to be used as a base film for magnetic recordingmedia and film capacitors.

The present inventors have made earnest studies for dissolving thesubjects described above and have accomplished the present inventionbased on the finding that use of a biaxially oriented film of a singlelayer or a laminate layer using an aromatic polyester and a polyolefinhaving a melting point of from 230 to 280° C. at a predetermined ratiocan decrease the dimensional change to the humidity change whilemaintaining the mechanical characteristic although the thickness isthin.

That is, the invention provides a biaxially oriented film of a singlelayer or a laminate layer comprising a aromatic polyester (a) and apolyolefin (b) having a melting point of from 230 to 280° C., in which

the ratio of the polyolefin (b) is within a range from 2 to 60% byweight based on the entire weight of the film and the film thickness iswithin a range from 1 to 10 μm.

Further, the invention includes a magnetic recording medium and a filmcapacitor using the biaxially oriented film as described above.

EFFECT OF THE INVENTION

In the biaxially oriented film of the invention, dimensional changes tothe changes of humidity are within a predetermined range although thethickness is thin. Accordingly, the biaxially oriented film of theinvention can be used suitably as a base film for magnetic recordingmedia.

Further, since the magnetic recording medium of the invention causesless track deviation and is excellent in increase of density andcapacitance, it is suitable, particularly, as a magnetic recordingmedium for use in data storage.

Further, the biaxially oriented film of the invention, has a dimensionalchange to the humidity changes within a predetermined range and has anexcellent withstand voltage characteristic. Accordingly, the biaxiallyoriented film of the invention can be used suitably as a base film forfilm capacitors.

Further, the film capacitor according to the invention is thin-walled,excellent in the withstand voltage characteristic and suitable as filmcapacitors for use in electric and electronic equipment and automobileparts requiring size reduction and heat resistance.

PREFERRED EMBODIMENTS THE INVENTION

(Biaxially Oriented Film)

The biaxially oriented film of the invention is a single layered film ora laminate film and, specifically, it includes, for example, theconstitution to be described later. It is necessary for the biaxiallyoriented film of the invention that it comprises an aromatic polyester(a) and a polyolefin (b) having a melting point of from 230 to 280° C.,and that the ratio of the polyolefin (b) is within a range from 2 to 60%by weight based on the entire weight of the film. In a case where thecontent of the polyolefin (b) is less than the lower limit, improvementfor the dimensional stability to the humidity change is not sufficient.Further, in a case where the content of the polyolefin (b) exceeds theupper limit, the obtained biaxially oriented film is poor in themechanical characteristic. A preferred ratio of the polyolefin (b) isfrom 3 to 55% by weight, more preferably, from 3 to 50% by weight,further preferably, from 5 to 50% by weight, and, particularlypreferably, from 5 to 30% by weight. In a case where the ratio of thepolyolefin (b) is less than the lower limit, improvement for thedimensional stability to the humidity change is not sufficient and, inaddition, the withstand voltage characteristic are not sometimesimproved sufficiently. Further, in a case where the ratio of thepolyolefin (b) exceeds 50% by weight, film formation under stretchingmay sometimes become difficult.

In the biaxially oriented film of the invention, the film thickness isnecessarily within a range from 1 to 10 μm and it is, preferably, from 2to 10 μm, further preferably, from 2 to 7 μm and, particularlypreferably, from 3 to 7 μm. In a case where the thickness exceeds theupper limit, the film thickness become excessive to shorten the tapelength to be contained in a cassette in a case of use, for example, formagnetic recording media and no sufficient magnetic recording capacitycan be obtained. Further, in a case of use for capacitors, it isdifficult to reduce the size of the capacitor. On the other hand, belowthe lower limit, since the film thickness is thin, film breakage occursfrequently during film preparation, or the winding property of the filmbecomes sometimes poor.

(Aromatic Polyester (a))

The aromatic polyester (a) in the invention is a polymer obtained bypolycondensation of a diol and an aromatic dicarboxylic acid. Thearomatic dicarboxylic acid includes, for example, terephthalic acid,isophthalic acid, 2,6-naphthalene dicarboxylic acid, and4,4′-diphenyldicarboxylic acid, and the diol includes, for example,ethylene glycol, 1,4-butanediol, 1,4-cyclohexane dimethanol, and1,6-hexanediol. Among them, with a view point of the mechanicalcharacteristic and the heat resistance, polyethylene terephthalate andpolyethylene-2,6-naphthalene dicarboxylate are preferred, andpolyethylene-2,6-naphthalene dicarboxylate is preferred.

The polyester resin in the invention may be used alone, as a copolymerwith other polyester, or a mixture of two or more kinds of polyesters,and it is used preferably alone, from a view point of the mechanicalcharacteristic and the heat resistance. Other ingredient in thecopolymer or the mixture is, preferably, 10 mol % or less and, morepreferably, 5 mol % or less based on the number of mols for therepetitive structural units. The copolymerization ingredient includes adiol ingredient such as diethylene glycol, neopentyl glycol, andpolyalkylene glycol, and dicarboxylic acid ingredient such as adipicacid, sebacic acid, phthalic acid, isophthalic acid, terephthalic acid,and 5-sodium sulfoisophthalic acid.

The intrinsic viscosity of the polyester resin in the invention is,preferably, 0.40 or more and, more preferably, 0.40 to 0.80 ino-chlorophenol, at 35° C. In a case where the intrinsic viscosity isless than 0.4, cutting occurs frequently during film formation, orstrength of products after molding fabrication sometimes becomesinsufficient. On the other hand, in a case where the intrinsic viscosityexceeds 0.8, productivity upon polymerization is lowered.

The melting point of the polyester resin in the invention is,preferably, from 200 to 300° C., more preferably, from 240 to 300° C.and, particular preferably, from 260 to 290° C. In a case where themelting point is lower than the lower limit, the heat resistance of thepolyester film is sometimes insufficient. Further, in a case where themelting point exceeds the upper limit, mixing with the polyolefin (b)sometimes become difficult.

The dielectric constant of the polyester resin in the invention ispreferably from 2.7 to 3.4 under the condition at 23° C., 1 MHz. Suchdielectric constant is a characteristic inherent to the polyester resin.

(Polyolefin (b))

The polyolefin in the invention (hereinafter sometimes referred to aspolyolefin (b)) is a polyolefin having a melting point of from 230 to280° C. The polyolefin includes, for example, poly-3-methylbutene-1,poly-4-methylpentene-1, polyvinyl-t-butane,1,4-trans-poly-2,3-dimethylbutadiene, polyvinyl cyclohexane,polystyrene, polymethylstyrene, polydimethylstyrene, andpolybutylstyrene. Among them, from a view point of the heat resistanceand the mechanical characteristic, a styrene polymer having asyndiotactic structure (hereinafter sometimes referred to as asyndiotactic styrene polymer) is preferred.

The syndiotactic styrene polymer in the invention is a polystyrenehaving a syndiotactic structure in view of a stereochemical structureand the tacticity measured by nuclear magnetic resonance method (¹³C-NMRmethod) is 75% or more and, preferably, 85% or more for a diad(constituent unit: 2) and 30% or more and, preferably, 50% or more for apentad (constituent unit: 5).

The syndiotactic styrene polymer includes polystyrene,poly(methyl)styrene, poly(ethylstyrene), poly(propylstyrene) andpoly(butylstyrene) as poly(alkylstyrene), and poly(phenylstyrene). Amongthem, polystyrene, poly(p-methylstyrene), poly(m-methylstyrene), andpoly(p-tertiary butylstyrene) can be preferably mentioned as examples.The syndiotactic styrene polymer in the invention may be used alone ortwo or more of them may be used in combination.

Further, the polymerization average molecular weight of the syndiotacticstyrene polymer in the invention is preferably, 10,000 or more and, morepreferably, 50,000 or more. In a case where the polymerization averagemolecular weight is less than the lower limit, heat resistance andmechanical characteristic are insufficient. On the other hand, the upperlimit of the polymerization average molecular weight is preferably500,000 or less. In a case where it exceeds the upper limit, it issometimes poor in the film forming property.

The melting point of the polyolefin in the invention is preferably from240 to 275° C. In a case where the melting point is lower than the lowerlimit, mixing with the aromatic polyester (a) is difficult and the heatresistance of the obtained biaxially oriented film is insufficientsometimes. Also in a case where the melting point exceeds the upperlimit, mixing with the aromatic polyester becomes difficult.

The polyolefin in the invention has a dielectric constant under thecondition at 23° C., 1 MHz of less than 3.0, more preferably, within arange from 2.2 to 2.9. In a case where the dielectric constant exceedsthe upper limit, the withstand voltage characteristic of the biaxiallyoriented film is not sometimes improved sufficiently. Further, in a casewhere the dielectric constant is less than the lower limit, thepolyolefin is sometimes poor in the fabricability.

The polyolefin in the invention preferably has dielectric loss of lessthan 0.001. The dielectric loss is represented by dielectric tangent(tan δ) under the condition at 23° C., 1 MHz. In a case where thedielectric loss is 0.001 or more, the insulation property isdeteriorated and the withstand voltage characteristic of the obtainedbiaxially oriented film is not sometimes improved sufficiently.

(Single Layered Film)

The biaxially oriented film of the invention includes a single layeredfilm as a preferred form. The single layered film is preferably formedof a thermoplastic resin composition (c) as a mixture of the aromaticpolyester (a) and the polyolefin (b). In the single layered film, theratio of the aromatic polyester (a) and the polyolefin (b) is from 40 to98% by weight of the aromatic polyester (a) and from 2 to 60% by weightof the polyolefin (b) based on the weight of the thermoplastic resincomposition (c) forming the film. The content of the aromatic polyester(a) is, preferably, from 45 to 97% by weight, more preferably, from 50to 97% by weight, further preferably, from 50 to 95% by weight and,particularly preferably, from 70 to 95% by weight. In a case where thecontent of the aromatic polyester is less than the lower limit, theobtained biaxially oriented film is sometimes poor in the mechanicalcharacteristic. Further, in a case where it is less than 50% by weight,the film formation by stretching can not sometimes be improvedsufficiently. On the other hand, in a case where the content of thearomatic polyester exceeds the upper limit, improvement in thedimensional stability to the humidity change is not sometimesinsufficient and the withstand voltage characteristic is not sometimessufficient.

Further, in the thermoplastic resin composition (c), the content of thepolyolefin (b) is, preferably, from 3 to 55% by weight, more preferably,from 3 to 50% by weight, further preferably, from 5 to 50% by weight,and particularly preferably, from 5 to 30% by weight. In a case wherethe content of the polyolefin (b) is less than the lower limit,improvement for the dimensional stability to the humidity change is notsometimes sufficient and the withstand voltage characteristic is notsometimes improved sufficiently. On the other hand, in a case where thecontent of the polyolefin (b) exceeds the upper limit, the obtainedbiaxially oriented film is sometimes poor in the mechanicalcharacteristic. Further, in a case where it exceeds 50% by weight, filmformation by stretching sometimes becomes difficult.

(Laminate Film)

(Biaxially Oriented Film(X))

The biaxially oriented film of the invention includes a laminate film asa preferred form. Such a biaxially oriented film is preferably abiaxially oriented film (X) comprising a film layer A formed from athermoplastic resin composition (c′) as a mixture of the aromaticpolyester (a) and the polyolefin (b), and a film layer B comprising anaromatic polyester (a) laminated at least on one surface thereof. In thefilm layer A, it is further preferred that the ratio of the aromaticpolyester (a) and the polyolefin (b) is within a range described belowbased on the weight of the thermoplastic resin composition (c′) formingthe film layer A. That is, in the thermoplastic resin composition (c′),the aromatic polyester (a) ranges from 5 to 95% by weight, preferably,from 7 to 93% by weight, more preferably, from 10 to 90% by weight, andparticularly preferably, from 50 to 90%, and the polyolefin (b) rangesfrom 5 to 95% by weight, preferably, from 7 to 93% by weight, morepreferably, from 10 to 90% by weight, and particularly preferably, from10 to 50% by weight. In the thermoplastic resin composition (c′), in acase where the content of the aromatic polyester (a) exceeds the upperlimit, or the content of the polyolefin (b) is less than the lowerlimit, the intended effect of improving the dimensional stability to thehumidity change is poor. On the other hand, in a case where the contentof the aromatic polyester (a) is less than the lower limit, or thecontent of the polyolefin (b) exceeds the upper limit, the obtainedbiaxially oriented laminate film is poor in the mechanicalcharacteristic. In a case where the content of the aromatic polyester(a) exceeds 50% by weight, particularly excellent film forming propertycan be obtained and the adhesion with the film layer B is increased.

Further, the thickness of the film layer A relative to the thickness ofthe laminate film is within a range, preferably from 5 to 95%, morepreferably, from 7 to 93% and, particularly preferably, from 10 to 90%.In a case where the thickness of the film layer A is less than the lowerlimit, the effect of improving the dimensional stability to the humiditychange is poor and, on the other hand, in a case where the thickness ofthe film layer A exceeds the upper limit, the obtained biaxiallyoriented film is poor in the mechanical characteristic.

It may suffice that the film layer B is a film layer comprisingsubstantially the aromatic polyester (a) and may also contain otherthermoplastic resin, for example, the polyolefin (b) within a range notdeteriorating the purpose of the invention. The content of the aromaticpolyester (a) in the film layer B is preferably 90% by weight or moreand, more preferably, 95% by weight or more based on the weight of thefilm layer B.

The existent amount of the polyolefin (b) in the biaxially oriented film(X), based on the weight of the laminate film is within a range from 2to 60% by weight, preferably, from 3 to 55% by weight, more preferably,from 3 to 50% by weight, further preferably, from 5 to 50% by weightand, particularly preferably, from 5 to 30% by weight. In a case wherethe existent amount of the polyolefin (b) is less than the lower limit,the aimed effect of improving the dimensional stability to the humiditychange is sometimes poor and the withstand voltage characteristic is notsometimes sufficient. On the other hand, in a case where the existentamount of the polyolefin (b) exceeds the upper limit, the obtainedbiaxially oriented film is sometimes poor in the mechanicalcharacteristic. In a case where it exceeds 50% by weight, film formationby stretching sometimes becomes difficult.

The biaxially oriented film (X) includes as a preferred layerconstitution, for example, (i) a 2-layered constitution in which a filmlayer B is laminated on one surface of the film layer A, (ii) a3-layered constitution in which the film layer B is laminated on bothsurfaces of the film layer A, and (iii) a multi-layered structure inwhich the film layer A and the film layer B are laminated at least by 4layers as the number for the entire layers. In the case of the 3-layeredconstitution (ii), curling resistance is further improved. Further, in acase of the multi-layered constitution (iii), a laminate of film layerscomprising different kinds of resins can be formed into a film withoutworsening steps due to inter-layer delamination, etc. In a case of themulti-layered constitution (iii), a preferred number for the entirelayers is 8 layers or more, more preferably, 16 layers or more and,particularly preferably, 32 layers or more. While the upper limit is notparticularly restricted, it is about 500 layers and, preferably, 250layers with a point of preventing complexity of steps. In this case, thefilm layer A and the film layer B are preferably laminated alternately,and a film layer comprising other resin may also be laminated within arange not deteriorating the purpose of the invention. In the case of themulti-layered constitution (iii), the thickness per one layer of thefilm layer A is preferably within a range from 0.02 to 1.5 μm and,further preferably, from 0.04 to 1.0 μm. On the other hand, thethickness per one layer of the film layer B is within a range,preferably, from 0.02 to 1.5 μm and further preferably, from 0.04 to 1.0μm. In a case where the thickness per one layer of the film layer A orthe film layer B is less than the lower limit, it is necessary tolaminate an extremely large number of layers, tending to make the stepcomplicate. On the other hand, in a case where the thickness per onelayer of the film layer A or the film layer B exceeds the upper limit,inter-layer delamination may sometimes occur. The thickness describedabove can be measured by cutting the laminate film in the direction ofthe thickness by a microtome or the like into a micro slice piece andobserving the same under transmission electron microscope.

(Biaxially Oriented Film (Y))

As another preferred form of the laminate film, a biaxially orientedfilm (Y) having a film layer B comprising the aromatic polyester (a) anda film layer C comprising the polyolefin (b) laminated at least on onesurface thereof is preferred.

In the biaxially oriented film (Y), the film layer B comprises thearomatic polyester (a), and may be mixed or copolymerized with otherresin in such a range as not deteriorating the purpose of the invention.The content of the aromatic polyester (a) in the film layer B is,preferably, 90% by weight or more and, more preferably, 95% by weight ormore based on the weight of the film layer B.

In the biaxially oriented (Y), the film layer C comprises the polyolefin(b) and may be mixed or copolymerized with other resin within such arange as not deteriorating the purpose of the invention. The content ofthe polyolefin (b) in the film layer C, is preferably, 90% by weight ormore and, more preferably, 95% by weight or more based on the weight ofthe film layer C.

The existent amount of the polyolefin (b) in the biaxially oriented film(Y), based on the weight of the laminated film, is within a range offrom 2 to 60% by weight, preferably, from 3 to 55% weight, morepreferably, from 3 to 50% by weight, further preferably, from 5 to 50%by weight, and, particularly preferably, 5 to 30% by weight. In a casewhere the existent amount of the polyolefin (b) is lower than the lowerlimit, the intended effect of improving the dimensional stability to thehumidity change is sometimes poor, and the withstand voltagecharacteristic is not sometimes sufficient. On the other hand, in a casewhere the existent amount of the polyolefin (b) exceeds the upper limit,the obtained biaxially oriented film is sometimes poor in the mechanicalcharacteristic. Further, in a case where it exceeds 50% by weight, thefilm formation under stretching becomes sometimes difficult.

The biaxially oriented film (Y) include as a preferred layerconstitution, for example, (i) a 2-layered constitution in which a filmlayer B is laminated on one surface of the film layer C, (ii) a3-layered constitution in which the film layer B is laminated on bothsurfaces of the film layer C, and (iii) a multi-layered structure inwhich the film layer C and the film layer B are laminated at least by 4layers as the number for the entire layers. In the case of the 3-layeredconstitution (ii), curling resistance is further improved. Further, in acase of the multi-layered constitution (iii), a laminate of film layerscomprising different kinds of resins can be formed into a film withoutworsening steps due to inter-layer delamination, etc. In a case of themulti-layered constitution (iii), a preferred number for the entirelayers is 8 layers or more, more preferably, 16 layers or more and,particularly preferably, 32 layers or more. While the upper limit is notparticularly restricted, it is about 500 layers and, preferably, 250layers with a point of preventing complexity of steps. In this case, thefilm layer B and the film layer C are preferably laminated alternately,and a film layer comprising other resin may also be laminated within arange not deteriorating the purpose of the invention. In the case of themulti-layered constitution (iii), the thickness per one layer of thefilm layer B is, preferably, within a range from 0.02 to 1.5 μm and,further preferably, from 0.04 to 1.0 μm. On the other hand, thethickness per one layer of the film layer C is within a range,preferably, from 0.02 to 1.5 μm and further preferably, from 0.04 to 1.0μm. In a case where the thickness per one layer of the film layer B orthe film layer C is less than the lower limit, it is necessary tolaminate an extremely large number of layers tending to make the stepcomplicate. On the other hand, in a case where the thickness per onelayer of the film layer B or the film layer C exceeds the upper limit,inter-layer delamination may sometimes occur. The thickness describedabove can be measured by cutting the laminate film in the direction ofthe thickness by a microtome or the like into a micro slice piece andobserving the same under transmission electron microscope.

The biaxially oriented film in the invention includes, as a specificexample, the single layered film and the laminate film described above,a specific example of the laminate film includes a biaxially orientedfilm (X) and the biaxially oriented film (Y), and a layer constitutionsuitable to a required characteristic can be used further. Among thelayer constitutions described above, the single layered film or thebiaxially oriented film (X) is preferred with a view point of layerdelamination. Particularly, in a case of the single layered film,excellent dimensional stability to the humidity change due to thepolyolefin (b) and excellent mechanical characteristic and film formingproperty due to the aromatic polyester (a) can be developed since thisis in the form of a mixture. Further, in a case of the single layeredfilm of the invention, a withstand voltage characteristic equal withthat of the polyolefin (b) can be provided even when the blending amountof the polyolefin (b) is small. Further, among the biaxially orientedfilm (X), the excellent mechanical characteristic and the film formingproperty due to the aromatic polyester (a) can be developed easily in acase of the 2-layered constitution due to the further lamination of thefilm layer B. Further, with a view point of the curling resistance, the3-layered constitution of the biaxially oriented film (X) is preferred.

(Dispersion State of Film Layer Comprising Thermoplastic ResinComposition)

The film layer comprising the thermoplastic resin composition (c) or thethermoplastic resin composition (c′) of the invention is preferablyformed of a mixture of the aromatic polyester (a) and the polyolefin (b)in which the polyolefin (b) is dispersed in an island shape. “Islanddispersion shape” may be any of circular, elliptic or bar-like shape. Itis more preferred in the invention that many bar-like dispersion shapesextended in the MD direction are observed and the average length of theMD direction is 20 μm or less. For the average length, the cross sectionin the direction of thickness parallel with the MD direction of theobtained film was observed by using an optical microscope (OPTPHOT-2,manufactured by Nikon Co.) under 200 X and measuring the length in theMD direction of the dispersion phase comprising the olefin (b) by thenumber of 100, to determine an average length.

The average length in the MD direction is, more preferably, 15 μm orless and, particularly preferably, 10 μm or less. In a case where theaverage length exceeds the upper limit, breakage tends to occur in thefilm stretching step. Further, as the film thickness is reduced, effectof the size of the dispersion phase becomes significant and breakagetends to occur in the film stretching step.

The method of rendering the average length in the MD direction to 20 μmor less includes a physical method by a kneading method and a chemicalmethod such as by using a compatibilizing agent. It is more preferred tofurther comprising a compatibilizing agent to the thermoplastic resincomposition (c) or the thermoplastic resin composition resin (c′) sincethis can be coped with an existent apparatus.

The compatibilizing agent includes those having a function of decreasingthe size of the dispersion phase comprising the polyolefin (b), inaddition to the definition for usual compatibilizing agents. While thereis no particular restriction so long as they have the function describedabove and they include, for example, a thermoplastic amorphous resin (d)having a solubility parameter (hereinafter sometimes referred to simplyas SP value) between the aromatic polyester (a) and the polyolefin (b).The SP value of the aromatic polyester (a) or the polyolefin (b) isdetermined depending on the kind and the copolymerization ingredient ofthe resin to be used. For examples, the SP value calculated by a Fedormethod (hereinafter simply referred to as a Fedor method) is 23.6(MJ/m³)^(0.5) for polyethylene terephthalate, 24.8 (MJ/m³)^(0.5) (Fedormethod) for polyethylene-2,6-naphthalate in the aromatic polyester (a),and 20.7 (MJ/m³)^(0.5) (Fedor method) for polystyrene in the polyolefin(b).

The thermoplastic amorphous resin (d) includes, for example, acrylicacid copolymerized polyolefin and vinyl oxazoline copolymerizedpolyolefin resins and the monomer constituting the olefin ingredient inthe copolymer is further preferably styrene. Further, as the monomerconstituting the acrylic acid ingredient in the copolymer includes, forexample, acrylic acid, methacrylic acid, methyl(meth)acrylate,ethyl(meth)acrylate, and butyl(meth)acrylate. For enhancing thecompatibilizing effect, an epoxy group may further be introduced to thethermoplastic amorphous resin (d).

The thermoplastic amorphous resin (d) is contained preferably within arange from 0.1 to 10% by weight based on the weight of the thermoplasticresin composition (c) or the thermoplastic resin composition (c′). Thecontent of the thermoplastic amorphous resin (d) is, more preferably,from 0.2 to 5% by weight and, particularly preferably, from 0.3 to 3% byweight. In a case where the content is less than the lower limit, sincethe effect as the compatibilizing agent does not develop, the averagelength of the polyolefin (b) is not within the desired range and thefilm forming property is not sometimes improved. On the other hand, in acase where the content exceeds the upper limit, gelation due tocrosslinking reaction may sometimes occur.

It is preferred that the thermoplastic resin composition (c) or thethermoplastic resin composition (c′) of the invention does not havevoids. The voids mean voids formed at the boundary between the aromaticpolyester (a) forming the matrix phase and the polyolefin (b) formingthe island phase. The void can be determined in the same method as thatfor determining the average length of the polyolefin (b) by using theoptical microscope (OPTPHOT-2, manufactured by Nikon Co.) under 200×.Further, “does not have voids” means a state in the observation by theoptical microscope described above that the number of dispersion phaseswhere voids are observed around the dispersion phase is 10 or less, morepreferably, 5 or less among the dispersion phases comprising the olefin(b) by the number of 100.

In a case where voids are present, the films sometimes tend to be cut inthe film stretching step. Further, as the film thickness is reduced, theportions for the voids result in defects to sometimes deteriorate themechanical characteristic or deteriorate the withstand voltagecharacteristic.

In order not to have the voids, it is preferred that the polyolefin (b)having Tg lower than the glass transition point (Tg) of the aromaticpolyester (a) is selected, and that the stretching temperature of thefilm is Tg or higher for the aromatic polyester (a). Further, it ispreferred to use a resin having a compatibility parameter approximate tothat of the aromatic polyester among the polyolefins. Further, the voidscan be eliminated also by containing the compatibilizing agent.

(Inert Particles)

The biaxially oriented film of the invention can contain inert particlesin the film, for example, inorganic particles containing elements ofgroup IIA, IIB, IVA, and IVB of the periodical table (for example,kaolinite, alumina, titanium oxide, calcium carbonate, and silicondioxide), and fine particles comprising a highly heat resistant polymerssuch as particles of crosslinked silicone resin, crosslinkedpolystyrene, crosslinked acrylic particles.

In a case of incorporating the inert particles, the average particlediameter of the inert particles is preferably within a range from 0.001to 5 μm and they are preferably contained within a range from 0.01 to10% by weight based on the entire weight of the film.

In a case of use for magnetic recording media, a preferred averageparticle diameter and a content of the inert particles are asexemplified below depending on the single layered film or the laminatefilm.

That is, in a case of using the single layered film for the magneticrecording media, the average particle diameter of the inert particle is,preferably, from 0.01 to 1.0 μm, more preferably, from 0.03 to 0.8 μmand, particularly, from 0.05 to 0.6 μm. The content of the inertparticles is, preferably, from 0.01 to 1.0% by weight, more preferably,from 0.03 to 0.8% by weight and, particularly preferably, from 0.05 to0.5% by weight, based on the weight of the thermoplastic resincomposition (c). Further, while the inert particles contained in thefilm may be either of a single ingredient system or multi-ingredientsystem, it is preferred to contain inert particles of 2-ingredientsystem or higher multi-ingredient system from a view point ofcompatibilizing the electromagnetic conversion characteristic of thetape and the winding property of the film. The surface roughness of thefilm surface (WRa) is preferably controlled by properly selecting theaverage particle diameter and the addition amount of the inert particleswithin the range described above.

In a case of using the biaxially oriented film (X) or the biaxiallyoriented film (Y) as the laminate film for the magnetic recording media,the average particle diameter of the inert particles is, preferably,from 0.01 to 0.8 μm, more preferably, from 0.02 to 0.6 μm and,particularly preferably, from 0.03 to 0.4 μm. Further, referring to thecontent of the inert particles, the inert particles are not contained inthe surface on the side of the planar surface or, if contained, it is atmost 0.5% by weight, preferably, 0.4% by weight and, more preferably,0.3% by weight based on the weight of the film layer forming the surfaceon the side of the planar surface. On the other hand, for the surface onthe side of the rough surface, it is preferred that the inert particlesare contained by, preferably, from 0.01 to 1.0% by weight, preferably,from 0.03 to 0.8% by weight and, particularly preferably, from 0.05 to0.6% by weight based on the weight of the film layer forming the roughsurface. In a case of 4 or more layered laminate structure, a film layercomprising the composition identical with that of the film layer formingthe surface on the side of the rough surface may contain identical inertparticles with those for the surface layer on the side of the roughsurface. Further, while the inert particles contained in the film may beeither of a single ingredient system or multi-ingredient system, it ispreferred to contain inert particles of 2-ingredient system or highermulti-ingredient system from a view point of compatibilizing theelectromagnetic conversion characteristic of the tape and the windingproperty of the film. The surface roughness of the film surface (WRa) ispreferably controlled by properly selecting the average particlediameter and the addition amount of the inert particles within the rangedescribed above.

Further, in a case of the single layered film, the roughness on onesurface and that on the other surface can not be changed easily, but ina case of the laminate film, the roughness on one surface and that onthe other surface can be changed easily to provide an advantage that theelectromagnetic conversion characteristic and the film running propertycan be compatibilized easily.

(Additive)

The biaxially oriented film in the invention may optionally contain asmall amount of UV-absorbent, antioxidant, antistatic agent, lightstabilizer, and heat stabilizer.

Further, the biaxially oriented film in the invention may also contain aphosphorous compound. There is no particular restriction for the kind ofthe phosphorous compound so long as it is a phosphorus compound thatfunctions as the heat stabilizer and includes, for example, phosphoricacid, phosphate esters such as methyl phosphate and ethyl phosphate,phosphorous acid and phosphite esters, and among the phosphoruscompounds, triethyl phosphono acetate can be mentioned particularlypreferably.

A preferred content of the phosphorus compound is from 20 to 300 ppm,further, from 30 to 250 ppm and particularly, from 50 to 200 ppm as amolar concentration of the elemental phosphorus in the phosphoruscompound to the entire dicarboxylic acid ingredient of the polyester. Ina case where the content of the phosphorus compound is less than thelower limit, the ester exchange reaction catalyst is not deactivatedcompletely to sometimes worsen the heat stability and deteriorate thedynamic characteristic. On the other hand, in a case where the contentof the phosphorus compound exceeds the upper limit, it sometimes worsensthe heat stability and deteriorates the mechanical characteristic.

(Humidity Expansion Coefficient in the Width Direction)

The biaxially oriented film of the invention has a humidity expansioncoefficient αh in the transverse direction of the film preferably withina range from 0.1×10⁻⁶ to 13×10⁻⁶/% RH. A more preferred αh is from0.5×10⁻⁶ to 11×10⁻⁶/% RH and, particularly preferably, within a rangefrom 0.5×10⁻⁶ to 10×10⁻⁶/% RH.

In a case of decreasing the αh to less than the lower limit, thepolyolefin (b) is present excessively, which may sometimes lower thefilm forming property or deteriorate the mechanical characteristic. Onthe other hand, in a case where the upper limit is exceeded, the filmelongates due to humidity change to sometimes bring about trackdeviation or the like when used for the magnetic recording media.Further, in a case of use for film capacitors, the capacitorcharacteristic is not sometimes sufficient in the use requiring highhumidity circumstance such as in an automobile engine room. Such αh canbe attained by improving the Young's modulus in the direction ofmeasurement by stretching and mixing the polyolefin together. In a casenot stretched in the width direction, since the Young's modulus in thewidth direction is low, the humidity expansion coefficient within therange described above can not be obtained even when the polyolefinsmixed together.

(Temperature Expansion Coefficient in Width Direction)

The biaxially oriented film of the invention has a temperature expansioncoefficient αt in the width direction of the film preferably within arange from −10×10⁻⁶ to +15×10⁻⁶/° C. A preferred αt is from −8×10⁻⁶ to+10⁻⁶/° C. and, particularly, with a range from −5⁻⁶×10⁻⁶ to +5×10⁻⁶/°C. In a case where αt is less than the lower limit, it causes shrinkageand, on the other hand, in a case where it exceeds the upper limit, thefilm elongates by the temperature change to sometimes bring about trackdeviation or the like when used for magnetic recording media. Further,in a case where the temperature expansion coefficient αt exceeds theupper limit, the capacitor characteristic is not sometimes sufficient inthe use for capacitors requiring a high temperature circumstance such asin automatic engine rooms. Such αt can be attained by improving theYoung's modulus in the measuring direction by stretching and, settingthe existent amount of the polyolefin to less than the upper limitdescribed above. In a case where it is not stretched in the widthdirection, since the Young's modulus in the width direction is low, thetemperature expansion coefficient within the range described above cannot be obtained even when the polyolefin is present being mixedtogether.

(Young's Modulus)

The biaxially oriented film of the invention preferably has Young'smodulus of 5 GPa or more both in the film forming direction (MDdirection) and the width direction (hereinafter sometimes referred to asthe transverse direction or TD direction) of the film. In a case wherethe Young's modulus is less than the lower limit in any one of thedirections, it can not sometimes endure the applied load when used asmagnetic recording media or it is deformed by the temperature/humiditychange even when the dimensional change by the humidity change is small.Further, the sum for the Young's modulus in the film forming directionand the width direction is preferably 22 GPa at the greatest. In a casewhere the sum of the Young's modulus in the film forming direction andthe Young's modulus in the width direction exceeds the upper limit, thestretching factor increase excessively during film formation tosometimes result in film breakage frequently or worsen the yield ofproducts remarkably. A preferred upper limit for the sum of Young'smodulus in the film forming direction and in the width direction is 20GPa or less and, further, 18 GPa or less.

By the way, in a case of use for linear track type magnetic tapes, it ispreferred that the Young's modulus in the film forming direction islarger than the Young's modulus in the width direction with a view pointof decreasing elongation in the film forming direction. A preferredYoung's modulus is such that the Young's modulus in the film formingdirection is larger than the Young's modulus in the width direction, theYoung's modulus in the film forming direction is 6 GPa or more, 7 GPa ormore and, particularly, 8 GPa or more, and the Young's modulus in thewidth direction is 5 GPa or more, further, 6 GPa or more and,particularly, 7 GPa or more. Further, it is preferred that the Young'smodulus in the width direction is larger than the Young's modulus in thefilm forming direction from a view point of extremely decreasing theelongation in the width direction. A preferred Young's modulus is suchthat the Young's modulus in the width direction is larger than theYoung's modulus in the film forming direction, the Young's modulus inthe width direction is 7 GPA or more, 8 GPa or more and, particularly, 9GPa or more, and the Young's modulus in the film forming direction is 5GPa or more, further, 6 GPa or more and, particularly, 7 GPa or more.Furthermore, it is preferred that the difference of the Young's modulusin the film forming direction and the Young's modulus in the widthdirection is 2 GPa or less, particularly, 1 GPa or less, the Young'smodulus in the film forming direction is 6 GPa or more, 7 GPa or moreand, particularly, 8 GPa or more and the Young's modulus in the widthdirection is 6 GPa or more, further, 7 GPa or more and, particularly, 8GPa or more, from the view point of decreasing the elongation both inthe film forming direction and the width direction.

(Breakdown Voltage)

The biaxially oriented film of the invention has an breakdown voltagepreferably in excess of 400 V/μm. The breakdown voltage is, morepreferably, 410 V/μm or more, further preferably, 460 V/μm or more and,particularly preferably, 470 V/μm or more. In a case where the breakdownvoltage is the lower at or less than limit, the electric characteristicwhen used for capacitors is not sometimes sufficient. The breakdownvoltage is a value measured at a DC current of 160 V/s by usingITS-6003, name of the apparatus, manufactured by Tokyo Seiden Co.,according to a plate electrode method specified in JIS C2151.

(Heat Resistant Temperature)

The biaxially oriented film of the invention preferably has a heatresistant temperature of 110° C. or higher. The heat resistanttemperature is, more preferably, 115° C. or higher and, particularlypreferably, 120° C. or higher. In a case where the heat resistanttemperature is lower than the lower limit, the heat resistance is notsometimes insufficient when used for capacitors. The heat resistanttemperature is defined as a temperature capable of withstanding for20000 hours based on the Arrhenius plot for a relation between thehalf-decay time of an breakdown voltage and the temperature according tothe temperature index of IEC 60216.

(Coating Film Layer)

The biaxially oriented film of the invention may also have a coatingfilm layer on at least one surface of the outermost layer (hereinaftersometimes referred to as a coating layer). The coating film layer isobtained by coating a coating agent comprising a binder resin and asolvent to a biaxially oriented film. As the binder resin, various kindsof resins of thermoplastic resins or thermosetting resins can be usedand they include, for example, polyester, polyimide, polyamide,polyester amide, polyolefin, polyvinyl chloride, poly(meth)acrylic acidester, polyurethane, and polystyrene, as well as copolymers or mixturesthereof. Among the binder resins, the polyester copolymer is aparticularly preferred example. The solvent includes, for example,organic solvents such as toluene, ethyl acetate, and methyl ethyl ketoneand mixtures thereof and, further it may be water.

The coating film layer in the invention may further containcrosslinkers, surfactants and inert particles as the ingredients forforming the coating film. The surfactants include, for example,polyalkylene oxides.

In addition to the ingredient described above, the coating film layer inthe invention may further contain other resins such as melamine resin,flexible polymer, filler, heat stabilizer, weather stabilizer,anti-aging agent, labeling agent, antistatic agent, slipping agent,anti-blocking agent, anti-clouding agent, dye, pigment, natural oil,synthetic oil, wax, emulsifier, hardener, and flame retardant, and theblending ratio is properly selected within such a range as notdeteriorating the purpose of the invention.

The method of laminating the coating film layer on the biaxiallyoriented film in the invention may be either a method of coating anddrying a coating agent on at least one surface of a biaxially stretchedfilm, or a method of coating a coating agent to a stretchable film, thendrying, stretching, and optionally applying a heat treatment. Thestretchable film is a non-stretched film, a monoaxially stretched film,or a biaxially stretched film and, among them, a longitudinallystretched film stretched monoaxially in the film extruding direction(longitudinal direction) is exemplified particularly preferably.

Further, in a case of coating the coating agent to the film, coating ina clean atmosphere, that is, coating in the film forming step ispreferred, which improves the adhesion of the coating membrane to thefilm. In a case of coating to the heat set film during a usual coatingstep, that is, in a step separated from the film manufacturing stepafter biaxial stretching, dirt or dust are tended to be involved.

As the method of coating the coating agent to the film, any of knowncoating methods can be used and, for example, a roll coating method,gravure coating method, roll brushing method, spray coating method, airknife coating method, dipping method, and curtain coating method can beused alone or in combination.

(Surface Layer)

The biaxially oriented film of the invention may also be a laminate inwhich a further layer is laminated on at least one surface with an aimof providing another function.

For example, in a case of use for magnetic recording media, a polyesterfilm layer not substantially containing inert particles may be laminatedto the surface of a biaxially oriented film of the invention on the sideof a magnetic layer for making the side of the magnetic layer to a moreplanar surface. Further, for making the side of the running surface(nonmagnetic layer) to a surface more excellent in the running property,a polyester film layer in which the inert particles to be contained ismade relatively larger or increased in the amount may be laminated tothe surface of the biaxially oriented film of the invention on the sideof the non-magnetic layer. Such a laminate film is preferred in that theelectromagnetic conversion characteristic and the winding property ofthe film can be easily made compatible when formed as a magneticrecording medium.

Further, in a case of use for film capacitors, a layer D containing anoxygen atom-containing compound may be further provided on at least onesurface of a biaxially oriented film with an aim of, for example,further improving the self-healing property. The ratio of oxygen atomsto carbon atoms on the surface measured by an X-ray photoelectronspectroscopy is preferably 10% or more and, more preferably, 15% ormore. In a case where the (oxygen atom/carbon atom) ratio is less thanthe lower limit, the self-healing property under voltage applicationsometimes becomes poor. The oxygen atom containing compound includes,for example, cellulose and SiO₂. In the case of cellulose, it can belaminated by a method of coating while incorporating cellulose within arange from 5 to 50% by weight in the binder ingredient of the coatingfilm layer. In the case of SiO₂, lamination can be made by any of themethods of vacuum vapor deposition, ion plating, or sputtering.

Further, the thickness of the layer D containing the oxygenatom-containing compound is preferably 30% or less to the entirethickness of the film. In a case where it is 30% more in the thickness,the electric characteristic such as temperature, frequencycharacteristic of electrostatic capacitance and dielectric tangentsometimes become poor. There is no particular restriction on the lowerlimit of the thickness, in a case where it is less than 0.005 μm, theeffect of improving the self healing property is sometimes difficult tobe obtained.

(Surface Roughness WRa)

The biaixally oriented film of the invention preferably has a surfaceroughness WRa (center surface average roughness) suitable to usedepending on the application use.

For example, in a case of use for magnetic recording media, the surfaceroughness WRa (center plane average roughness) for one surface of thebiaxially oriented films is preferably from 1 to 10 nm, further, from 2to 10 nm and, particularly, from 2 to 8 nm. As the surface roughness WRais more than 10 nm, the surface of the magnetic layer becomes rough andsatisfactory electromagnetic conversion characteristic can not beobtained sometimes. On the other hand, in a case where the surfaceroughness WRa is less than 1 nm, since the surface becomes excessivelyplanar, slipping on a pass roll or calendar is worsened to sometimescause wrinkles, failing to efficient coating of the magnetic layer orfailing to efficient calendaring.

Further, the surface roughness WRa on the other surface may be identicalwith WRa described above, or may be larger than WRa described above andfrom 5 to 20 nm, further, from 6 to 15 nm and, particularly, from 8 to12 nm. In a case where the surface roughness WRa on the other surface islarger than the upper limit, the unevenness at the surface on the sideof the running surface is transferred to the surface on the side of themagnetic layer to roughen the surface on the side of the magnetic layerthereby sometimes failing to obtain a satisfactory electromagneticconversion characteristic. On the other hand, in a case where thesurface roughness WRa is less than the lower limit, the surface becomesexcessively planar to sometimes worsen the slipping on a pass roll orcalendar, result in wrinkles and fail to effectively coat magneticlayer. In the case of the biaxially oriented laminate film, it ispreferred to make the two surfaces into different forms of surface sincethe electromagnetic conversion characteristic and the running propertycan be controlled more easily.

The surface roughness WRa described above can be controlled byincorporating inert particles in the film, or by a surface treatmentforming fine unevenness, for example, by a coating treatment of acoating agent.

Further, in a case of use for film capacitors, the surface roughness WRa(center plane average roughness) of the biaxially oriented film is,preferably, from 1 to 150 nm, further, from 10 to 120 nm and,particularly, from 30 to 100 nm. In a case where the surface roughnessWRa is larger than the upper limit, upon fabrication into a capacitor,protrusions on the film are excessively large to make the dielectriccharacteristic instable due to air present between the films, or theprotrusions tend to sometimes lower the breakdown voltage. On the otherhand, in a case where the surface roughness WRa is less than the lowerlimit, the film is excessively planar to possibly cause disadvantagessuch as in the operation efficiency in the metal vapor deposition stepand the film winding step, deformation and adhesion between the films inthe capacitor heat treatment step and the pressing step and, as aresult, the capacitance of the capacitor sometimes varies greatly.

(Film Forming Method)

The biaxially oriented film of the invention is preferably manufacturedby the following method.

The biaxially oriented film of the invention can be manufactured, in acase of the single layered film, by using the aromatic polyester (a) andthe polyolefin (b) described above as the starting material, extrudingthem in a molten state into a sheet-like shape, and by using knownfilm-forming methods such as tenter method, inflation method or thelike, and it includes, for example, a method of mixing the aromaticpolyester (a) and the polyolefin (b) each in a predetermined amount,drying, then supplying them to an extruder heated to 300° C. and moldinginto a sheet-like shape by a T die.

It can be manufactured preferably by using a method of extruding at atemperature from a melting point of an aromatic polyester (Tm: ° C.) to(Tm+70)° C., and rapidly cooling to solidify into a not-extended filmand further stretching the not-stretched film in a monoaxial direction(longitudinal direction or transverse direction) by a predeterminedfactor at a temperature from (Tg−10) to (Tg+70)° C., then stretching ata predetermined factor in the direction perpendicular to the stretchingdirection described above (in a case where the first step is in thelongitudinal direction, the second step is in the transverse direction)at a temperature from Tg to (Tg+70)° C. and, further, applying a heattreatment. In this case, the stretching factor, the stretchingtemperature, the heat treatment condition, etc. are selected and decideddepending on the characteristic of the film. The area stretching factoris, preferably, from 6 to 35 times and, in a case of use for capacitors,from 6 to 25 times, more preferably, from 7 to 16 times. Further, in acase for use in magnetic recording media, it is preferably by from 15 to35 times and, further, from 20 to 30 times. The heat setting temperaturemay be determined within a range from 190 to 250° C. and the treatmenttime may be determined within a range from 1 to 60 sec. Particularly, ina case where the heat resistance is necessary, it is preferred to applyheat setting in a range from 210 to 240° C. in order to improve thedimensional stability under high temperature condition. By conductingsuch heat setting treatment, the heat shrinkage of the obtainedbiaxially oriented film at 200° C. can be from −3.5 to 3.5%, morepreferably, −3 to 3% and, particularly preferably, from 0 to 3%. In acase where the heat shrinkage is within the ranges described above,wrinkles less occur to the film when fabricated into capacitors.Further, for suppressing the heat shrinkage, after applying a heattreatment at 150 to 220° C. for 1 to 60 sec in an off-line step, anannealing treatment of gradually cooling in a temperature atmosphere offrom 50 to 80° C. may also be applied.

In addition to the sequential biaxial stretching method, a simultaneousbiaxial stretching method may also be used. In the sequential biaxialstretching method, the number of stretching in the longitudinaldirection or the transverse direction is not restricted to once butlongitudinal-transverse stretching can be conducted by stretchingtreatment for several times and is not restricted in view of the numberof times. For example, in a case for use in magnetic recording, when itis intended to further improve the mechanical property, it is preferredfor subjecting the biaxially stretched film before the heat settingtreatment to a heat treatment at a temperature from (Tg+20) to (Tg+70)°C., further, stretching the same in the longitudinal direction or thetransverse direction at a temperature higher by 10 to 40° C. than theheat treatment temperature, successively stretching it further in thetransverse direction or the longitudinal direction at a temperaturehigher by 20 to 50° C. than the stretching temperature thereby settingthe total stretching factor to 3.0 to 7.0 times in the longitudinaldirection and setting the total stretching factor to 3.0 to 6.0 times inthe transverse direction.

In a case of manufacturing a 2-layered or 3-layered laminate film, amethod by co-extrusion is mentioned. Preferably, starting materialsconstituting the respective layers are laminated in a molten state by aco-extrusion method in a die and then extruded into a sheet-like shape,or two or more kinds of molten polyesters are extruded from a die andlaminated, and rapidly quenched to solidify into a laminatednot-stretched film and then subjected to biaxial stretching and heattreatment by the same method and under the same condition as those forthe case of the single layered film to form a laminated biaxiallyoriented film.

In a case of manufacturing four- or more layered laminate film, it canbe manufactured, for example, by a simultaneous multi-layer extrusionmethod using a feed block as proposed in JP-A No. 2000-326467, paragraph0028. That is, after drying an aromatic polyester (a) constituting thefilm layer B and a thermoplastic resin composition (c′) constituting thefilm layer A or a polyolefin (b) constituting the film layer C, they aresupplied to an extruder heated to about 300° C., and each of moltenproducts is, for example, laminated alternately by using a feed blockand spread in a die and extruded to form a not-stretched laminated filmand then it is subjected to biaxial stretching and heat treatment by thesame method under the same condition as those in the case of the singlelayered film to form a laminated biaxially oriented film.

Further, in a case of providing a coating layer, it is preferred to coata desired coating solution on one surface or both surfaces of thenot-stretched film or monoaxially stretched film described above.

(Magnetic Recording Medium)

The invention provides a magnetic recording medium using the biaxiallyoriented film of the invention described above as a base film and havinga magnetic layer on one surface thereof.

The magnetic recording medium is not particularly restricted so long asthe biaxially oriented film of the invention is used as the base filmand includes, for example, linear track system data storage tapes suchas QIC or DLT, and, S-DLT or LTO of a further higher capacity type.Since the dimensional change of the base film due to thetemperature/humidity change is extremely small, a magnetic recordingmedium suitable to high density and high capacity causing less trackdeviation can be provided even when the track pitch is narrowed in orderto ensure the high capacity of the tape.

(Film Capacitor)

According to the invention, a film capacitor using the biaxiallyoriented film of the invention as a base film and having a metal layeron at least one surface thereof is provided. The material for the metallayer is not particularly restricted and includes, for example,aluminum, zinc, nickel, chromium, tin, copper and alloys thereof.Further, in a case of providing a layer D containing an oxygenatom-containing compound for improving the self-healing property, theconstitution of the film capacitor includes, for example, a basefilm/layer D/metal layer, or layer D/base film/metal layer.

The film capacitor is not particularly restricted so long as thebiaxially oriented film of the invention is used as the base film andused for electric equipment, for example, in electric and electronicuses requiring miniaturization, or in driver's cabin or in an engineroom requiring heat resistance and humidity resistance for automobileuse. Further, since the base film has extremely small dimensional changeupon temperature/humidity change and is excellent in the heat resistanceand the voltage withstanding characteristic represented by the breakdownvoltage, the film capacitor can be further miniaturized and usedsuitably at high temperature and high humidity.

EXAMPLE

The present invention is to be described with reference to examples.Respective characteristic values and the evaluation methods weremeasured and evaluated by the following methods. Parts and % in theexamples mean parts by weight and % by weight, respectively.

(1) Melting Point, Glass Transition Point

10 mg of an aromatic polyester (a) or a polyolefin (b) was sealed in analuminum pan used for measurement, which was measured at a temperatureelevation rate of 20° C./min from 25° C. to 300° C. by using adifferential scanning calorimeter DSC 2920 manufactured by TAinstruments Co. to determine respective melting points (melting point ofthe aromatic polyester (a): Tma, melting point of the polyolefin (b):Tmb), and the glass transition point (glass transition point of thearomatic polyester (a): Tga, glass transition point of the polyolefin(b): Tgb).

(2) Average Particle Diameter of Inert Particles

Measurement is conducted by using a CP-50 type Centrifugal Particle SizeAnalyzer manufactured by Shimazu Seisakusho Co. From an accumulationcurve for the particle diameter of each particle and the existent amountthereof calculated based on the obtained centrifugal settling curve, aparticle diameter corresponding to 50 mass % is read and the value isdefined as the average particle diameter.

(3) Humidity Expansion Coefficient (αh)

A film sample is cut out to 15 mm length and 5 mm width such that thewidth direction of the film is along the measuring direction, which isset to TMA 3000 manufactured by Shinku Riko Inc. and kept in anatmosphere at 30° C. at constant humidity 30% RH and humidity 70% RH ina nitrogen atmosphere, the length of the specimen is measured in thiscase to calculate the humidity expansion coefficient according to theequation (1) described below. The measuring direction is along thelongitudinal direction of a specimen and measurement is conducted on thespecimens by the number of 10 and the average value thereof is definedas αh.αh=(L ₇₀ −L ₃₀)/(L ₃₀ ×ΔH)  (1)in which

L₃₀: specimen length (mm) at 30% RH

L₇₀: specimen length (mm) at 70% RH

ΔH: 40 (=70−30)% RH

(4) Temperature Expansion Coefficient (αt)

A film sample is cut out to 15 mm length and 5 mm width such that thewidth direction of the film is along the measuring direction, which isset to TMA 3000 manufactured by Shinku Riko Inc., applied with apretreatment in a nitrogen atmosphere (0% RH), at 60° C. for 30 min andthen the temperature is lowered to the room temperature. Then, thetemperature is elevated from 25° C. to 70° C. at 2° C./min, the specimenlength at each temperature is measured and the temperature expansioncoefficient (αt) is calculated according to the equation (2) describedbelow. The measuring direction is along the longitudinal direction ofthe specimen, measurement is conducted 10 specimens and an average valuethereof is used.αt={(L ₆₀ −L ₄₀)/(L ₄₀ ×ΔT)}+0.5×10⁻⁶  (2)in which L₄₀: specimen length (mm) at 40° C.

-   L₆₀ specimen length (mm) at 60° C.-   ΔT: 20 (=60−40)° C.-   0.5×10⁻⁶: temperature expansion coefficient of quartz glass    (5) Young's Modulus

A film is cut out to 10 mm width and 15 cm length as a specimen andpulled by an instron type universal tensile tester with a chuck distanceof 100 mm, at a tensile speed of 10 mm/min and a chart speed of 500mm/min, and the Young's modulus is calculated from a tangent at therising portion of the obtained load-elongation curve.

The measuring direction is along the longitudinal direction of thespecimen, and the Young's modulus is measured by 10 times and an averagevalue thereof is used.

(6) Surface Roughness (WRa)

A center surface average roughness (WRa) is determined according to theequation (3) described below using a non-contact 3-dimensional roughnessmeter (NT-2000) manufactured by WYKO Co., under the conditions at ameasuring factor of 25, a measuring area of 246.6 μm×187.5 μm (0.0462mm²) by a surface analysis software incorporated in the roughness meter.Measurement is repeated by 10 times and an average value thereof isused.

$\begin{matrix}{{WRa} = {\sum\limits_{k = 1}^{M}{\sum\limits_{j = 1}^{N}{{{Z_{jk} - \overset{\_}{Z}}}/\left( {M \cdot N} \right)}}}} & (3)\end{matrix}$in which

$\overset{\_}{Z} = {\sum\limits_{k = 1}^{M}{\sum\limits_{j = 1}^{N}{Z_{jk}/\left( {M \cdot N} \right)}}}$

Z_(jk) is a height on the three dimensional roughness chart at the jthand kth positions when the measuring direction (246.6 μm) and thedirection perpendicular thereto (187.5 μm) are divided by M and dividedby N, respectively.

(7) Thickness for Each Film

A laminate film is cut out to a trigonal shape, fixed in an embeddingcapsule and then embedded with an epoxy resin. It is cut in thedirection parallel with the film forming direction and the direction ofthe thickness by a microtome (ULTRACUT-S) into a thin slice of 50 nmthickness. Then, it is observed by using a transmission type electronmicroscope at an acceleration voltage of 1000 kV, photographed at amagnification factor of 10000 to 100000×, and the thickness for each ofthe layers was measured by photograph.

(8) Dispersibility of Olefin (b) and Void

The cross sectional thickness parallel with the MD direction of a filmsample is observed by using an optical microscope (OPTPHOT-2,manufactured by Nikon Co.) at 200×, and the length in the MD directionof a dispersion phases comprising the olefin (b) by the number of 100 todetermine an average length.

Further, voids around the dispersion phase comprising the olefin (b) inthis case are measured and the number of dispersion phases where voidsare generated is determined among the dispersion phases comprising theolefin (b) by the number of 100 and it is judged based on the followingcriteria.

-   ◯: dispersion phases having voids are 10 or less.-   x: dispersion phases having voids exceed 10.    (9) Heat Resistance

Film samples are used and temperature durable for 20000 hrs isdetermined based on Arrhenius plots for the relation between the time ofthe half-decay time of the breakdown voltage and the temperatureaccording to the temperature index of IEC 60216.

(10) Dielectric Constant

Dielectric constant at 23° C., 1 MHz is measured by using athermoplastic resin and according to JIS C2151.

(11) Dielectric Loss

Dielectric loss at 23° C., 1 MHz is measured by using a thermoplasticresin and according to JIS C2151.

(12) Breakdown Voltage

Film samples are used and breakdown voltage is measured at a DC currentof 160 V/s using ITS-6003 manufactured by Tokyo Seiden Co. according tothe plate electrode method specified in JIS C 2151.

(13) Film Curling Property

Film samples are used, which are sampled into 30 mm length×200 mm width,and 200 mm length×30 mm width, spontaneously left on a flat plate,respectively, and judged visually in this state by the followingcriteria:

-   ◯: curling is scarcely observed-   Δ: curling is observed slightly-   x: curling occurs remarkably    (14) Peeling Resistance

To one surface of a sample film, 6 cuts are formed the number of 6 ineach of longitudinal and transverse directions at an interval of 2 mmtherebetween by a cutter knife to prepare 25 grids. Then, a pressuresensitive adhesive tape of 24 mm width (trade name of products,CELLOTAPE (registered trade mark) manufactured by Nichiban Co.) isappended on both surfaces of the sample film and, after peeling theadhesive tape on the side of the grids rapidly at a peeling angle of180°, the peeled surface was observed and evaluated by the followingcriteria:

-   ◯: adhesion between layers is satisfactory, with no peeled area,-   Δ: adhesion between the layers is poor with less than 20% of peeled    area-   x: adhesion between the layers is extremely poor, with more than 20%    of peeled area    (15) Film Forming Property

The state upon film formation was observed and ranked by the followingcriteria:

-   ◯◯: continuous film formation is possible for 12 hours or more with    no problems such as disconnection in view of film formation,-   ◯: long roll can be sampled, with the condition capable of film    formation being restricted-   x: film can be formed only for an extremely short period of time,    with the continuous film forming property being poor    (16) Track Deviation

After recording at temperature and humidity of 10° C., 10% RH by using adriving unit, LTO1 manufactured by Hewlett-Packard Co., it is run undertemperature and humidity at 30° C. and 80% RH and a track deviationwidth of the magnetic tape to the magnetic head due to the change oftemperature and humidity is measured.

A smaller absolute value for the deviation width means a betterproperty.

(17) Humidity Resistance of Capacitor

A voltage at 100V (DC) was applied under temperature and humidity at 60°C. and 95% RH to conduct aging for 500 hours by using 4192 A LFIMPEDANCE ANALYZER, manufactured by Hewlett-Packard Co., to measure thechange coefficient of static capacitance of capacitors and conductevaluation by the following criterion. In this case, the static capacitychange coefficient is represented by Δ C/C (%), in which C represents astatic capacitance before aging and ΔC is a absolute value obtained bysubtracting the static capacity before aging from the static capacitanceafter aging.

-   ◯: ΔC/C (%) is 5 or less-   x: ΔC/C (%) exceeds 5

Comparative Example 1

After conducting ester exchange reaction for dimethylnaphthalene-2,6-dicarboxylate and ethylene glycol by a customary methodunder the presence of manganese acetate, triethyl phosphono acetate wasadded. Then, antimony trioxide was added and polycondensation wasconducted by a customary method to obtain a polyethylene-2,6-nahpthalenedicarboxylate resin (intrinsic viscosity 0.62 (in orthochlorophenol at35° C.), hereinafter simply referred to as PEN)). As a result ofmeasuring the concentration for each of elements in this resin by atomicabsorption method, Mn=50 ppm, Sb=300 ppm, and P=50 ppm. In PEN, siliconeparticles of an 0.5 μm average particle diameter was added by 0.02% byweight, and silica particles of 0.1 μm average particle diameter wasadded by 0.3% by weight previously based on the weight of the resincomposition in the polymerization stage.

After drying the obtained PEN at 180° C. for 6 hours, it was supplied toan extruder heated to 300° C., extruded by using a T-extrusion die andsolidified by rapid cooling on a casting drum surface finished at 0.3 Sand kept at a surface temperature of 60° C. to obtain a not-stretchedfilm. The not-stretched film was preheated at 75° C., further heatedbetween low speed and high speed rolls by an infrared heater at asurface temperature of 830° C. from a portion 14 mm above and stretchedby 5.1 times in the film forming direction, cooled rapidly, successivelysupplied to a stenter, stretched by 4.8 times in the transversedirection at 125° C. Successively, after heat setting at 240° C. for 10sec, it was applied with a 1.0% relaxation treatment in the transversedirection at 120° C. to obtain a biaxially oriented film of 4.5 μmthickness. The Young's modulus of the obtained film was 8 GPa in thelongitudinal direction and 6.5 GPa in the transverse direction.

On the other hand, a composition shown below was placed in a ball milland after kneading and dispersion for 16 hours, 5 parts by weight of anisocyanate compound (Desmodule L, manufactured by Bayer Co.) was addedand dispersed under high speed shearing for one hour to prepare amagnetic coating material.

Composition of the Magnetic Coating Material:

Acicular Fe particles 100 parts by weight Vinyl chloride-vinyl acetatecopolymer 15 parts by weight (Eslex 7A, manufactured by Sekisui ChemicalCo.) Thermoplastic polyurethane resin 5 parts by weight Chromium oxide 5parts by weight Carbon black 5 parts by weight Lecithin 2 parts byweight Fatty acid ester 1 part by weight Toluene 50 parts by weightMethyl ethyl ketone 50 parts by weight Cyclohexanone 50 parts by weight

The magnetic coating material was coated on one surface of the PEN filmat a coating thickness of 0.5 μm, then applied with an orientationtreatment in a DC magnetic field at 2,500 Gauss and, after heating anddrying at 100° C., applied with a super calendaring treatment (linepressure: 2,000 N/cm, temperature: 80° C.) and then taken up. The takenup roll was left in an oven at 55° C. for three days.

Further, a back coat layer coating material of the following compositionwas coated to 1 μm thickness on the other surface of the PEN film, driedand, further, cut into 12.65 mm (=½ inch), to obtain a magnetic tape.

Composition for Backcoat Layer:

Carbon black 100 parts by weight Thermoplastic polyurethane resin  60parts by weight Isocyanate compound  18 parts by weight (Collonate L,manufactured by Nippon Polyurethane Industry Co.) Silicone oil  0.5parts by weight Methyl ethyl ketone 250 parts by weight Toluene  50parts by weight

Characteristics of the obtained biaxially oriented film and the magnetictape are shown in Table 1 and Table 5.

Example 1

A biaxially oriented film of 4.5 μm thickness having Young's modulus inthe longitudinal direction of 8 GPa, and a Young's modulus in thetransverse direction of 6.5 GPa was obtained by changing the PEN ofComparative Example 1 to a thermoplastic resin composition (c1) formedby uniformly blending 90% by weight of PEN and 10% by weight of asyndiotactic polystyrene (grade: 130 ZC, manufactured by IdemitsuPetrochemical Co.) and changing the stretching factor. In thethermoplastic resin composition (c1), 0.02% by weight of siliconeparticles with an average particle diameter of 0.5 μm and 0.3% by weightof silica particles with an average particle diameter of 0.1 μm werepreviously added based on the weight of the thermoplastic resincomposition (c1).

To the obtained biaxially oriented film the same procedures as those inComparative Example 1 were repeated to prepare a magnetic tape.

The characteristics of the obtained biaxially oriented film and themagnetic tape are shown in Table 1.

Example 2

The same procedures as those in Example 1 were repeated except for usinga thermoplastic resin composition (c2) in which the content of thesyndiotactic polystyrene (grade; 130 ZC, manufactured by IdemitsuPetrochemical Co.) was changed from 10% by weight to 30% by weight andchanging the stretching factor.

The characteristics of the obtained biaxially oriented film and themagnetic tape are shown in Table 1.

Example 3

The same procedures as those in Example 1 were repeated except for usinga thermoplastic resin composition (c3) in which the content of thesyndiotactic polystyrene (grade; 130 ZC, manufactured by IdemitsuPetrochemical Co.) was changed from 10% by weight to 50% by weight andchanging the stretching factor.

The characteristics of the obtained biaxially oriented film and themagnetic tape are shown in Table 1.

Example 4

The same procedures as those in Example 2 were repeated except forchanging the stretching factor to obtain a biaxially oriented film of4.5 μm thickness with the Young's modulus in the longitudinal directionof 8 GPa and the Young's modulus in the transverse direction of 8 GPa.

The characteristics of the obtained biaxially oriented film on themagnetic film are shown in Table 1.

Example 5

The same procedures as those in Example 2 were repeated except forchanging the stretching factor to obtain a biaxially oriented film of4.5 μm thickness with the Young's modulus in the longitudinal directionof 5.5 GPa and the Young's modulus in the traverse direction of 12 GPa.

The characteristics of the obtained biaxially oriented film on themagnetic film are shown in Table 1.

Example 6

The same procedures as those in Example 1 were repeated except for usinga thermoplastic resin composition (c4) formed by changing the PENcontent from 90% by weight to 89% by weight, and adding 1% by weight ofan epoxy group-containing acrylic acid copolymerized polystyrene (ALFONEUG-4070 manufactured by Toa Gosei Co.), SP value: 21.5 (Fedor method) asa compatibilizing agent instead of the thermoplastic resin composition(c1). The SP value for PEN was 24.8 (Fedor method) and the SP value ofthe syndiotactic polystyrene was 20.7 (Fedor method).

The characteristics of the biaxially oriented film and the magnetic tapeare shown in Table 1.

Example 7

The same procedures as those in Example 2 were repeated except for usinga thermoplastic resin composition (c5) formed by changing the PENcontent from 70% by weight to 69% by weight, and adding 1% by weight ofan oxazoline group-containing polystyrene (EPOCROS RPS-1005,manufactured by Nippon Catalyst Co., SP value: 22.2 (Fedor method)) as acompatibilizing agent instead of the thermoplastic resin composition(c2). The SP value for PEN was 24.8 (Fedor method) and the SP value ofsyndiotactic polystyrene was 20.7 (Fedor method).

The characteristics of the biaxially oriented film and the magnetic tapeare shown in Table 1.

Comparative Example 2

The same procedures as those in Example 1 were repeated except for usinga thermoplastic resin composition (c6) formed by changing the content ofthe syndiotactic polystyrene (GRADE; 130 ZC, manufactured IdemitsuPetrochemical Co.) from 10% by weight to 70% by weight instead of thethermoplastic resin composition (c1), and changing the stretchingfactor.

The characteristics of the obtained biaxially oriented film and themagnetic tape are shown in Table 1.

TABLE 1 Comp. Comp. Example Example Example Example Example ExampleExample Example Example Unit 1 2 3 4 5 6 7 1 2 Film thickness μm 4.5 4.54.5 4.5 4.5 4.5 4.5 4.5 4.5 Polyolefin ratio wt % 10 30 50 30 30 10 30 070 Compatibilizing agent wt % 1 1 Young's modulus Film-forming directionGPa 8.0 8.0 8.0 8.0 5.5 8.0 8.0 8.0 8.0 Width direction GPa 6.5 6.5 6.58.0 12 6.5 6.5 6.5 6.5 Temperature expansion ppm/° C. 7 7 8 −2 −8 7 7 78 coefficient Humidity expansion ppm/% RH 11 9 6 6 4 11 9 12 4coefficient Track deviation ppm 862 705 548 367 100 870 705 940 391Film-forming property — ∘∘ ∘ ∘ ∘ ∘ ∘∘ ∘∘ ∘∘ x

Example 8

After conducting ester exchange reaction for dimethylnaphthalene-2,6-dicarboxylate and ethylene glycol by a customary methodunder the presence of manganese acetate, triethyl phosphono acetate wasadded. Then, antimony trioxide was added and polycondensation wasconducted by a customary method to obtain a polyethylene-2,6-nahpthalenedicarboxylate resin (PEN). As a result of measuring the concentrationfor each of elements in this resin by an atomic absorption method, Mn=50ppm, Sb=300 ppm, and P=50 ppm.

After drying a thermoplastic resin composition (c7) formed by uniformlyblending 90% by weight of the obtained PEN (intrinsic viscosity 0.62)and 10% by weight of a syndiotactic polystyrene (GRADE; 130 ZC,manufactured by Idemitsu Petrochemical Co.) at 180° C., it was suppliedto an extruder heated to 300° C., and molded from a die at 290° C. intoa sheet-like shape. Further, a not-stretched film obtained by cooling tosolidify the sheet by a cooling drum at a surface temperature of 60° C.was introduced to a group of rolls heated to 140° C., stretched in thelengthwise direction (longitudinal direction) by 3.6 times and thencooled by a group of rolls at 60° C.

Successively, while holding both ends of the longitudinally stretchedfilm by clips, it was introduced to a tenter and stretched in thedirection perpendicular to the longitudinal direction (transversedirection) by 4.0 times in an atmosphere heated to a highest widthstretching temperature of 150° C. Then, it was heat set in the tenter at220° C. for 5 sec and further, applied with a 1% heat relaxation at 200°C., then, gradually cooled uniformly to a room temperature to obtain abiaxially oriented film of 5 μm thickness. The Young's modulus of theobtained film was 6.0 GPa in the longitudinal direction and 6.5 GPa inthe transverse direction.

Aluminum was vacuum vapor deposited by 500 Å on one surface of theobtained biaxially oriented film and taken up into a tape of 4.5 mmwidth to form reels. The obtained reels were wound around being stackedto obtain a wound body, then pressed at 150° C. under 1 MPa for 5 min.Metallikon was flame-sprayed on both end faces to form an externalelectrode, and lead wires were welded to the metallikon to manufacture awound film capacitor.

The characteristics of the aromatic polyester (a) and the polyolefin (b)used and the characteristics of the obtained biaxially oriented film andthe capacitors are shown in Table 2.

Example 9

The same procedures as those in Example 8 were repeated except for usinga thermoplastic resin composition (c8) formed by changing the content ofthe syndiotactic polystyrene (grade; 130 ZC, manufactured by IdemitsuPetrochemical Co.) from 10% by weight to 30% by weight, instead of thethermoplastic resin composition (c7).

The characteristics of the obtained biaxially oriented film and the filmcapacitor are shown in Table 2.

Example 10

A thermoplastic resin composition (c9) formed by changing PEN to apolyethylene terephthalate resin (PET) was used instead of thermoplasticresin composition (c7), dried at 170° C. for 3 hours, and supplied to anextruder heated to 280° C., and molded from a die at 290° C. into asheet-like shape. Further, the not-stretched film obtained by cooling tosolidify the sheet by a cooling drum at a surface temperature of 20° C.was introduced to a group of rolls heated to 90° C., stretched in thelengthwise direction (longitudinal direction) by 3.6 times and thencooled by a group of rolls at 20° C.

Successively, while holding both ends of the longitudinally stretchedfilm by clips, it was introduced to a tenter and stretched in thedirection perpendicular to the longitudinal direction (transversedirection) by 4.0 times in an atmosphere heated to a highest lateralstretching temperature of 120° C. Then, it was heat set in the tenter at220° C. for 5 sec, further, applied with 1% heat relaxation at 200° C.,then, cooled to a room temperature to obtain a biaxially oriented filmof 5 μm thickness.

The characteristics of the aromatic polyester (a) and the polyolefin (b)used, and the characteristics of the obtained biaxially oriented filmand the film capacitor are shown in Table 2.

Comparative Example 3

The same procedures as those in Example 8 were repeated except for using100% by weight of PEN instead of the thermoplastic resin composition(c7) and not using the syndiotactic polystyrene.

The characteristics of the obtained biaxially oriented film and the filmcapacitor are shown in Table 2.

Comparative Example 4

The same procedures as those in Example 10 were repeated except forusing 100% by weight of PET instead of the thermoplastic resincomposition (c9) and not using the syndiotactic polystyrene.

The characteristics of the obtained biaxially oriented film and the filmcapacitor are shown in Table 2.

Comparative Example 5

The same procedures as those in Example 8 were repeated except for usinga thermoplastic resin composition (c10) formed by changing the contentof the syndiotactic polystyrene (GRADE; 130 ZC, manufactured by IdemitsuPetrochemical Co.) from 10% by weight to 90% by weight instead of thethermoplastic resin composition (c7).

The characteristics of the obtained biaxially oriented film and the filmcapacitor are shown in Table 2.

TABLE 2 Comp. Comp. Comp. Example Example Example Example ExampleExample Unit 8 9 10 3 4 5 Film thickness μm 5.0 5.0 5.0 5.0 5.0 5.0Polyolefin ratio Wt % 10 30 10 — — 90 Aromatic polyester (a) Kind PENPEN PEN PEN PET PEN Melting point (Tma) ° C. 270 270 260 270 260 270Glass transition ° C. 120 120 75 120 75 120 temperature (Tga)Polyolefin(b) Melting point (Tmb) ° C. 270 270 270 — — 270 Glasstransition ° C. 93 93 93 — — 93 temperature (Tgb) Dielectric constant2.6 2.6 2.6 — — 2.6 Dielectric loss 0.0002 0.0002 0.0002 — — 0.0002Temperature expansion ppm/° C. 7 7 7 7 7 8 coefficient Humidityexpansion ppm/% RH 11 9 11 12 12 2 coefficient Breakdown voltage V/μm460 480 410 400 450 490 Heat resistance ° C. 120 120 95 120 95 120 Void— ∘ ∘ x — — — Film-forming property — ∘∘ ∘∘ x ∘∘ ∘∘ x Humidityresistance — ∘ ∘ ∘ x x ∘ of capacitor

Example 11

The same procedures as those in Example 8 were repeated except for usinga thermoplastic resin composition (c11) formed by changing the PENcontent from 90% by weight to 89% by weight, and adding 1% by weight ofan oxazoline group-containing polystyrene (EPOCROS RPS-1005,manufactured by Nippon Catalyst Co., SP value: 22.2 (Fedor method)) as acompatibilizing agent instead of the thermoplastic resin composition(c7) and changing the film thickness from 5.0 to 3.0 μm. The SP valuefor PEN was 24.8 (Fedor method) and SP value for syndiotacticpolystyrene was 20.7 (Fedor method).

The characteristics of the biaxially oriented film are shown in Table 3.

Example 12

The same procedures as those in Example 11 were repeated except forusing a thermoplastic resin composition (c12) formed by changing thecontent of PEN from 89% by weight to 79% by weight and changing thecontent of the syndiotactic polystyrene (GRADE; 130 ZC, manufactured byIdemitsu Petrochemical Co.) from 10% by weight, to 20% by weight insteadof the thermoplastic resin composition (c11).

The characteristics of the obtained biaxially oriented film are shown inTable 3.

Example 13

The same procedures as those in Example 11 were repeated except forusing a thermoplastic resin composition (c13) formed by changing thekind of the syndiotactic polystyrene to 10 mol % methyl styrenecopolymerized syndiotactic polystyrene instead of the thermoplasticresin composition (c11) and coating a water soluble coating solution ofthe following composition as a D layer so as to be 20 nm thickness afterstretching and drying on one surface of a film after monoaxialstretching.

(Composition of Coating Layer)

Binder resin A: Isophthalic acid copolymerized PEN 50 wt % Binder resinB: Hydroxypropyl cellulose (HPC-SL, Nippon 40 wt % Soda Co.) Surfactant:Alkylnonylphenyl ether 10 wt %

The characteristics of the obtained biaxially oriented film are shown inTable 3.

Further, a film laminate vapor deposited with aluminum to 600 Åthickness on one surface of the obtained film sample was cut into asquare shape of 1 cm for each side, two sheets of them were stacked andfurther put between rubber plates of 2 cm for each side, and appliedwith a load of 2 kg. In this state, when a voltage was applied to thefilm laminate, to cause dielectric breakdown, self-healing property wasobserved.

Example 14

The same procedures as those in Example 12 were repeated except forusing a thermoplastic resin composition (c13) formed by changing thecontent of PEN from 79% by weight to 80% by weight and changing thecontent of the compatibilizing agent from 1% by weight to 0% by weightinstead of the thermoplastic resin composition (c12).

While it was attempted to obtain a biaxially oriented film of 3.0 μmthickness, cutting occurred extremely frequently during manufacture.

Comparative Example 6

The same procedures as those in Example 11 were repeated except forchanging the content of PEN from 89% by weight to 100% by weight and notusing the syndiotactic polystyrene and the compatibilizing agent,instead of the thermoplastic resin composition (c11). Thecharacteristics of the obtained biaxially oriented film are shown inTable 3.

TABLE 3 Comp. Example Example Example Example Example Unit 11 12 13 14 6Film thickness μm 3.0 3.0 3.0 3.0 3.0 Polyolefin ratio wt % 10 20 10 20— Compatibilizing agent wt % 1 1 1 — — Aromatic polyester (a) Kind PENPEN PEN PEN PET Melting point (Tma) ° C. 270 270 270 270 270 Glasstransition ° C. 120 120 120 120 120 temperature (Tga) Polyolefin(b) KindSPS SPS PMS-SPS SPS Melting point (Tmb) ° C. 270 270 247 270 — Glasstransition ° C. 93 93 95 93 — temperature (Tgb) Dielectric constant 2.62.6 2.6 2.6 — Dielectric loss 0.0002 0.0002 0.0002 0.0002 — Temperatureexpansion ppm/° C. 7 7 7 7 7 coefficient Humidity expansion ppm/% RH 1110 11 10 12 coefficient Breakdown voltage V/μm 460 480 460 — 380 Heatresistance ° C. 120 120 120 — 120 Average length of μm 10 15 9 30 —dispersion phase Film-forming property — ∘∘ ∘∘ ∘∘ x ∘∘

Example 15

After conducting ester exchange reaction for dimethylnaphthalene-2,6-dicarboxylate and ethylene glycol by a customary methodunder the presence of manganese acetate, triethyl phosphono acetate wasadded. Then, antimony trioxide was added and polycondensation wasconducted by a customary method to obtain a polyethylene-2,6-nahpthalenedicarboxylate resin (a) (hereinafter simply referred to as PEN (a)). Asa result of measuring the concentration for each of elements in thisresin by atomic absorption method, Mn=50 ppm, Sb=300 ppm, and P=50 ppm.

After drying a thermoplastic resin composition (c′1) obtained byuniformly blending 25% by weight of the obtained PEN (a) (intrinsicviscosity (in orthochlorophenol, at 35° C.) 0.62) and 75% by weight of asyndiotactic polystyrehe (b) (grade: 130 ZC, manufactured by IdemitsuPetrochemical Co.) and, PEN (a) at 180° C. for 6 hrs respectively, theywere supplied to an extruder heated to 300° C. and laminated in a dieand extruded such that the thermoplastic resin composition (c′1) formeda film layer A and the PEN (a) formed a film layer B, by using amulti-manifold type co-extrusion die and rapidly quenched to solidify ona casting drum surface finished at 0.3 S and kept at a surfacetemperature of 60° C. to obtain a not-stretched film. They were extrudedsuch that the film layer A was in contact with the casting drum, 0.15%by weight of silica particles of 0.3 μm average particle diameter and0.1% by weight of silica particles of 0.1 μm average particle diameterwere previously added to the PEN constituting the film layer B based onthe weight of the layer in the polymerization stage, and 0.1% by weightof silica particles of 0.1 μm average particle diameter was previouslyadded to the thermoplastic resin composition (c′1) constituting the filmlayer A based on the weight of the layer in the polymerization stage.

A biaxially oriented laminate film was obtained by repeating the sameprocedures as those in Comparative Example 1 except for changing thestretching factor for the not-stretched film. The Young's modulus of theobtained film was 8 GPa in the longitudinal direction and 6.5 PGa in thetransverse direction. The thickness of the film layer A and the filmlayer B in the laminate film was controlled depending on the dischargerate, which was 4 μm for the film layer A and 2 μm for the film layer B.

The same procedures as those in Comparative Example 1 were repeated tothe obtained biaxially oriented laminate film, to obtain a magnetictape.

The magnetic coating material was formed on the surface of the filmlayer A of the biaxially oriented laminate film, and the backcoat layercoating material was formed on the surface of the film layer B of thebiaxially oriented laminate film.

Further, the ratio of the polyolefin (% by weight) was determined basedon the specific gravity 1.36 g/cm³ of the PEN film and the specificgravity of 1.04 g/cm³ of the syndiotactic polystyrene film.

Characteristics of the obtained biaxially oriented laminate film and themagnetic tape are shown in Table 4.

Example 16

The same procedures as those in Example 15 were repeated except forusing, instead of thermoplastic resin composition (c′1), a thermoplasticresin composition (c′2) in which the content of the syndiotacticpolystyrene (grade; 130 ZC manufactured by Idemitsu Petrochemical Co.)was changed from 75% by weight to 10% by weight and changing thestretching factor.

The characteristics of the obtained biaxially oriented film and themagnetic tape are shown in Table 4.

Examples 17 to 19

The same procedures as those in Example 15 were repeated except forusing, instead of thermoplastic resin composition (c′1), a thermoplasticresin composition (c′3) in which the content of the syndiotacticpolystyrene (grade; 130 ZC manufactured by Idemitsu Petrochemical Co.)was changed from 75% by weight to 30% by weight and changing thestretching factor.

The characteristics of the obtained biaxially oriented film and themagnetic tape are shown in Table 4.

Example 20

The same procedures as those in Example 15 were repeated except foradding 0.1% by weight of silica particles of 0.1 μm average particlediameter to PEN constituting the film layer B previously in thepolymerization stage, using, instead of the thermoplastic resin (c′1)constituting the film A, a thermoplastic resin composition (c′4) inwhich the content of the PEN(a) was changed from 25% by weight to 24% byweight, and 1% by weight of an oxazoline group-containing polystyrene(EPOCROSS RPS-1005, manufactured by Nippon Catalyst Co.) was added as acompatibilizing agent and adding 0.15% by weight of silica particles of0.3 μm average particle diameter and 0.1% by weight of silica particlesof 0.1 μm average particle diameter previously in the polymerizationstage.

The magnetic coating material was formed on the surface of the filmlayer B of the biaxially oriented laminate film, and the back layercoating material was formed on the surface of the film layer A of thebiaxially oriented laminate film.

The characteristics of the obtained biaxially oriented laminate film andthe magnetic tape are shown in Table 4.

Example 21

The same procedures as those in Example 20 were repeated except forusing, instead of the thermoplastic resin composition (c′4), athermoplastic resin composition (c′5) in which the content of thesyndiotactic polystyrene (grade; 130 ZC, manufactured by IdemitsuPetrochemical Co.) was changed from 75% by weight to 10% by weight andthe kind of the compatibilizing agent was changed to the epoxygroup-containing acrylic acid copolymerized polystyrene (ALFONE UG-4070,manufactured by Toa Gosei Co.) and changing the stretching factor.

The characteristics of the obtained biaxially oriented laminated filmand the magnetic tape are shown in Table 4.

Example 22

The same procedures as those in Example 20 were repeated except forusing, instead of the thermoplastic resin composition (c′4), athermoplastic resin composition (c′6) in which the content of thesyndiotactic polystyrene (grade; 130 ZC, manufactured by IdemitsuPetrochemical Co.) was changed from 75% by weight to 30% by weight andchanging the stretching factor.

The characteristics of the obtained biaxially oriented laminated filmand the magnetic tape are shown in Table 4.

Example 23

The same procedures as those in Example 22 were repeated except forusing, instead of the 2-layered constitution of: film layer A/film layerB, a three layered constitution of: film layer B/film layer A/film layerB, changing the thickness for each of the film layers after biaxialstretching to 1.0 μm/4.0 μm/1.0 μm respectively and changing thestretching factor.

The characteristics of the obtained biaxially oriented laminate film andthe magnetic tape are shown in Table 5.

Comparative Example 7

The same procedures as those in Comparative Example 1 were repeatedexcept for using, instead of the PEN(a), a thermoplastic resincomposition (c′7) in which the content of the syndiotactic polystyrene(grade; 130 ZC, manufactured by Idemitsu Petrochemical Co.) of thethermoplastic resin composition (c′1) was changed from 75% by weight to70% by weight, and changing the stretching factor.

The characteristics of the obtained biaxially oriented laminated filmand the magnetic tape are shown in Table 5.

Comparative Example 8

The not-stretched film obtained in Example 23 was stretched by 5.1 timesin the film forming direction of the film to obtain a monoaxiallyoriented film stretched only in the film forming direction.

The characteristics of the obtained monoaxially oriented film are shownin Table 5.

TABLE 4 Example Example Example Example Example Example Example ExampleUnit 15 16 17 18 19 20 21 22 Layer constitution Laminate LaminateLaminate Laminate Laminate Laminate Laminate Laminate Film thicknesslayer A [μm] 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 layer B [μm] 2.0 2.0 2.02.0 2.0 2.0 2.0 2.0 Polyolefin ratio layer A wt % 75 10 30 30 30 75 1030 layer B wt % 0 0 0 0 0 0 0 0 Laminate film wt % 47 7 20 20 20 47 7 20Young's modulus Film forming direction GPa 8.0 8.0 8.0 8.0 5.5 8.0 8.08.0 Width direction Gpa 6.5 6.5 6.5 8.0 12 6.5 6.5 6.5 Total 14.5 14.514.5 16.0 17.5 14.5 14.5 14.5 Temperature expansion ppm/° C. 7 7 7 −2 −87 7 7 coefficient Humidity expansion ppm/% RH 6 11 10 7 6 6 11 10coefficient Track deviation ppm 548 888 783 428 140 548 888 783 Filmcurling property Δ ∘ Δ Δ Δ Δ ∘ Δ Film forming property — ∘ ∘∘ ∘ ∘ ∘ ∘ ∘∘∘

TABLE 5 Comparative Comparative Comparative Unit Example 23 Example 1Example 7 Example 8 Layer constitution Laminate Single layered Singlelayered Laminate Film thickness layer B [μm] 1.0 1.0 layer A [μm] 4.04.5 6.0 4.0 layer B [μm] 1.0 1.0 Polyolefin ratio layer A % 30 0 70 30layer B % 0 — — 0 Laminate film % 20 0 70 20 Young's modulus Filmforming direction GPa 8.0 8.0 8.0 8.0 Width direction Gpa 6.5 6.5 6.54.5 Total 14.5 14.5 14.5 12.5 Temperature expansion coefficient ppm/° C.7 7 7 25 Humidity expansion coefficient ppm/% RH 10 12 4 14 Trackdeviation ppm 783 940 391 Film curling property ◯ ◯ ◯ ◯ Film formingproperty — ◯ ◯◯ X ◯

Comparative Example 9

The same procedures as those in Comparative Example 1 were repeatedexcept for changing the film thickness from 4.5 μm to 6.0 μm afterbiaxial stretching.

The characteristics of the obtained biaxially oriented film and themagnetic tape are shown in Table 6 and Table 7.

Example 24

A polyethylene-2,6-naphthalene dicarboxylate resin (PEN) having anintrinsic viscosity (in orthochlorophenol, at 35° C.) of 0.62 andmelting point Tm of 269° C. with addition of 0.02 wt % of siliconeparticles of 0.5 μm average particle diameter and 0.3 wt % of silicaparticles of 0.1 μm average particle diameter was prepared as a resinfor the film layer B. Further, a thermoplastic resin composition (c′8)with addition of 0.02 wt % of silicone particles of 0.5 μm averageparticle diameter and 0.3 wt % of silica particles of 0.1 μm averageparticle diameter, which was a mixture at a weight ratio of 50:50 of PENhaving an intrinsic viscosity (orthochlorophenol, at 35° C.) of 0.62 anda melting point Tm of 269° C., with addition of 0.02 wt % of siliconeparticles of 0.5 μm average particle diameter and 0.3 wt % of silicaparticles of 0.1 μm average particle diameter, and dried at 160° C. for5 hours, and a syndiotactic polystyrene (B) (Grade: 130 ZC, manufacturedby Idemitsu Petrochemical Co.) dried at 100° C. for 3 hours was preparedas a resin for the film layer (A). The polymers of the film layers A andB were supplied to extruders and melted and, after branching the polymerof the film layer B into 25 layers and the polymer of the film layer Ainto 24 layers, they were merged by using a multi-layer feed blockdevice so as to laminate the layer A and the layer B alternately,introduced while being kept in a lamination state as it was to a die,and cast on a casting drum to prepare a not-stretched laminate sheet of49 layers in total where the layers A and the layers B were laminatedalternately. In this case, the ratio of the extrusion amount of thepolymers between the layer B and the layer A was controlled to 8:2, andthey were laminated such that the layer B was present on both of thesurfaces. The not-stretched laminate sheet extruded from the die wasrapidly cooled to solidify on a casting drum surface finished to 0.3 Sand kept at a surface temperature of 60° C. into a not-stretched film.

The same procedures as those in Comparative Example 1 were repeated tothe laminated not-stretched film except for changing the stretchingfactor, to obtain a biaxially oriented laminate film having a Young'smodulus of 8 GPa in the longitudinal direction and a Young's modulus of6.5 GPa in the transverse direction. The thickness of the film layer Aand the film layer B in the laminate film was controlled by thedischarge amount, in which the film layer B had a thickness per onelayer of 0.192 μm and 4.8 μm in total, while the film layer A had athickness per one layer of 0.050 μm and 1.2 μm in total.

Then, the same procedures as those in Comparative Example 1 wererepeated to obtain a magnetic recording medium.

The ratio of the polyolefin (wt %) was determined based on the specificgravity of 1.36 g/cm³ of the PEN film and the specific gravity of 1.04g/cm³ of the syndiotactic polyethylene film.

The characteristics of the obtained biaxially oriented laminate film andthe magnetic tape are shown in Table 6.

Example 25

The same procedures as those in Example 24 were repeated except forchanging the inert particles contained in the resin of the film layer Aand the film layer B to 0.1% by weight of the silica particles of 0.1 μmaverage particle diameter, changing the thermoplastic resin composition(c′8) to a thermoplastic resin composition (c′9) as a mixture at a 40:60ratio by weight of PEN and syndiotactic polystyrene, and changing thestretching factor and the discharge rate of each of the layers inExample 24, to obtain a biaxially stretched laminate film having aYoung's modulus of 8 GPa in the longitudinal direction, a Young'smodulus of 6.5 GPa in the transverse direction, the thickness per onelayer of the film layer B of 0.168 μm, the total for the thickness ofthe film layer B of 4.2 μm, the thickness per one layer of the filmlayer A of 0.075 μm and the total for the thickness of the film layer Aof 1.8 μm.

Then, the same procedures as those in Comparative Example 1 wererepeated to obtain a magnetic recording medium.

The characteristics of the obtained biaxially oriented laminate film andthe magnetic tape are shown in Table 6.

Example 26

Then, the same procedures as those in Example 24 were repeated exceptfor changing the thermoplastic resin composition (c′8) to athermoplastic resin composition (c′10) which was a mixture at a 60:40ratio by weight of PEN and syndiotactic polystyrene (B), and changingthe stretching factor and the discharge rate for each of the layers inExample 24, to obtain a biaxially oriented laminate film having aYoung's modulus of 8 GPa in the longitudinal direction, a Young'smodulus of 6.5 GPa in the transverse direction, a thickness per onelayer of the film layer B of 0.120 μm, the total thickness for the filmlayers B of 3.0 μm, the thickness per one layer of the film layers A of0.125 μm and the total thickness for the film layers A of 3.0 μm.

Then, the same procedures as those in Comparative Example 1 wererepeated to obtain a magnetic recording medium.

The characteristics of the obtained biaxially oriented laminate film andthe magnetic tape are shown in Table 6.

Example 27

The same procedures as those in Example 24 were repeated except forchanging the stretching factor and the discharge rate for each of thelayers, and changing the number of layers to 9 layers for the film layerB and to 8 layers for the film layer A, with the film layer B beingdisposed on both ends in Example 24, to obtain a biaxially orientedlaminate film having a Young's modulus of 8 GPa in the longitudinaldirection, a Young's modulus of 6.5 GPa in the transverse direction, andthe thickness per one layer of film B of 0.333 μm, the total thicknessfor the film layers B of 3.0 μm, the thickness per one layer of the filmA of 0.375 μm, and the total thickness of the film layers A of 3.0 μm.

Then, the same procedures as those in Comparative Example 1 wererepeated to obtain a magnetic recording medium.

The characteristics of the obtained biaxially oriented laminate film andthe magnetic tape are shown in Table 6.

Example 28

The same procedures as those in Example 24 were repeated except forchanging the stretching factor and the discharge rate for each of thelayers, and changing the number of layers to 49 layers for the filmlayer B and to 48 layers for the film layer A, with the film layer Bbeing disposed on both ends in Example 24, to obtain a biaxiallyoriented laminate film having a Young's modulus of 8 GPa in thelongitudinal direction, a Young's modulus of 6.5 GPa in the transversedirection, and the thickness per one layer of film B of 0.037 μm, thetotal thickness for the film layers B of 1.8 the thickness per one layerof the film A of 0.088 μm, and the total thickness for the film layers Aof 4.2 μm.

Then, the same procedures as those in Comparative Example 1 wererepeated to obtain a magnetic recording medium.

The characteristics of the obtained biaxially oriented laminate film andthe magnetic tape are shown in Table 6.

Example 29

The same procedures as those in Example 24 were repeated except forchanging the stretching factor and the discharge rate for each of thelayers in Example 26 to obtain a biaxially oriented laminate film havinga Young's modulus of 8 GPa in the longitudinal direction, and a Young'smodulus of 8 GPa in the transverse direction.

Then, the same procedures as those in Comparative Example 1 wererepeated to obtain a magnetic recording medium.

The characteristics of the obtained biaxially oriented laminate film andthe magnetic tape are shown in Table 6.

Example 30

The same procedures as those in Example 24 were repeated except forchanging the stretching factor and the discharge rate for each of thelayers in Example 26 to obtain a biaxially oriented laminate film havinga Young's modulus of 5.5 GPa in the longitudinal direction, and aYoung's modulus of 12 GPa in the transverse direction.

Then, the same procedures as those in Comparative Example 1 wererepeated to obtain a magnetic recording medium.

The characteristics of the obtained biaxially oriented laminate film andthe magnetic tape are shown in Table 6.

Comparative Example 10

The same procedures as those in Example 24 were repeated except forchanging the inert particles contained in the resin of the film layer Aand the film layer B to 0.02% by weight of silicone particles of 1.2 μmaverage particle diameter and 0.4% by weight of silica particles of 0.1μm average particle diameter, changing the thermoplastic resin (c′8) toa thermoplastic resin composition (c′11) which comprises only thesyndiotactic polystyrene (b), and changing the stretching factor and thedischarge rate for each of the layers, to obtain a biaxially orientedlaminate film having a Young's modulus of 8 GPa in the longitudinaldirection, a Young's modulus of 6.5 GPa in the transverse direction, athickness per one layer of the film layer B of 0.072 μm, the totalthickness for the film layers B of 1.8 μm, the thickness per one layerof the film layer B of 0.175 μm and the total thickness for the filmlayers A of 4.2 μm.

Then, the same procedures as those in Comparative Example 1 wererepeated to obtain a magnetic recording medium.

The characteristics of the obtained biaxially oriented laminate film andthe magnetic tape are shown in Table 6.

TABLE 6 Comp. Comp. Example Example Example Example Example ExampleExample Example Example Unit 24 25 26 27 28 29 30 9 10 Layerconstitution Multi- Multi- Multi- Multi- Multi- Multi- Multi- SingleMulti- layered layered layered layered layered layered layered layeredlayered Film thickness 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 Per one layerB μm 0.192 0.168 0.120 0.333 0.037 0.120 0.120 — 0.072 Total for layersB μm 4.8 4.2 3.0 3.0 1.8 3.0 3.0 6.0 1.8 Per one layer A μm 0.050 0.0750.125 0.375 0.088 0.125 0.125 — 0.175 Total for layers A μm 1.2 1.8 3.03.0 4.2 3.0 3.0 — 4.2 Number of layers layer B — 25 25 25 9 49 25 25 —25 layer A — 24 24 24 8 48 24 24 — 24 Total layer — 49 49 49 17 97 49 49— 49 Thickness ratio for layer Layer B/layer A — 8/2 7/3 5/5 5/5 3/7 5/55/5 10/— 3/7 Ratio of aromatic polyester Film layer A wt % 50 40 60 5025 60 60 — 0 Laminate film wt % 91 84 81 77 49 81 81 — 36 Ratio ofpolyolefin Film layer A wt % 50 60 40 50 75 40 40 — 100 Laminate film wt% 9 16 19 23 51 19 19 — 64 Young's modulus Film forming direction GPa8.0 8.0 8.0 8.0 8.0 8.0 5.5 8.0 8.0 Width direction GPa 6.5 6.5 6.5 6.56.5 8.0 12 6.5 6.5 Temperature expansion ppm/° C. 7 7 7 7 8 −2 −8 7 8coefficient Humidity expansion ppm/% RH 11 10 10 9 6 7 5 12 4coefficient Track deviation ppm 862 799 783 744 528 428 140 940 391Peeling resistance — ∘ ∘ ∘ ∘ ∘ ∘ ∘ — x Film forming property — ∘ ∘ ∘ ∘ ∘∘ ∘ ∘∘ x

Example 31

Polyethylene-2,6-naphthalene dicarboxylate resin (PEN) having anintrinsic viscosity (in orthochlorophenol at 35° C.) of 0.62 and amelting point (Tm) of 269° C., with addition of 0.02 wt % of siliconeparticles of 0.5 μm average particle diameter and 0.3 wt % of silicaparticles of 0.1 μm average particle diameter was prepared as a resinfor the film B. Further, a syndiotactic polystyrene (grade: 130 ZCmanufactured by Idemitsu Petrochemical Co.) with addition of 0.02 wt %of silicone particles of 0.5 μm average particle diameter and 0.3 wt %of silica particles of 0.1 μm average particle diameter was prepared asa resin for the film layer C. After drying the polymer for the filmlayer B at 160° C. for 3 hours and the polymer for the film layer C at100° C. for 3 hours, they were supplied to extruders and melted. Afterbranching the polymer of the film layer B into 25 layers and the polymerof the film layer C into 24 layers, they were merged by using amulti-layer feed block device so as to laminate the layer B and thelayer C alternately, introduced while being kept in a lamination stateas it was to a die, and cast on a casting drum to prepare not-stretchedlaminate sheet with the 49 layers in total where the layer B and thelayer C are laminated alternately. In this case, the ratio of theextruding amount of the polymer between the layer B and the layer C iscontrolled to 9:1, and they were laminated such that the layers B werepresent on both of the surfaces. The not-stretched laminate sheetextruded from the die was rapidly cooled to solidify on a casting drumsurface finished to 0.3 S and kept at a surface temperature of 60° C.into a not-stretched film.

The same procedures as those in Comparative Example 1 were repeatedexcept for changing the stretching factor to the not-stretched laminatefilm, to obtain a biaxially oriented laminate film having a Young'smodulus of 8 GPa in the longitudinal direction and a Young's modulus of6.5 GPa in the transverse direction. The thickness of the film layer Band the film layer C in the laminate film was controlled by thedischarge rate, in which the film layer B had a thickness per one layerof 0.216 μm and 5.4 μm in total, the film layer C had a thickness perone layer of 0.025 μm and 0.6 μm in total.

Then, the same procedures as those in Comparative Example 1 wererepeated to obtain a magnetic recording medium.

The ratio of polyolefin (wt %) was determined based on the specificgravity of 1.36 g/cm³ of the PEN film and the specific gravity of 1.04g/cm³ of the syndiotactic polyethylene film.

The characteristics of the obtained biaxially oriented laminate film andthe magnetic tape are shown in Table 7.

Example 32

Same procedures as those in Example 31 were repeated except for changinginert particles contained in the resin for the film layer B and the filmlayer C to 0.1% by weight of silica particles of 0.1 μm average particlediameter and changing the stretching factor and the discharge rate foreach of the layers in Example 31, to obtain a biaxially orientedlaminate film having a Young's modulus of 8 PGa in the longitudinaldirection and a Young's modulus of 6.5 GPa in the transverse direction,and the thickness per one layer of the film layer B of 0.168 μm, thetotal thickness for the film layer B of 4.2 μm, and the thickness perone layer of the film layer C of 0.075 μm and the total thickness forthe film layer B of 1.8 μm.

Then, the same procedures as those in Comparative Example 1 wererepeated to obtain a magnetic recording medium.

The characteristics of the obtained biaxially oriented laminate film andthe magnetic tape are shown in Table 7.

Example 33

The same procedures as those in Example 31 were repeated except forchanging the stretching factor and the discharge rate for each of thelayers in Example 31, to obtain a biaxially oriented laminate filmhaving a Young's modulus of 8 GPa in the longitudinal direction, aYoung's modulus of 6.5 GPa in the transverse direction, and thethickness per one layer of film layer B of 0.120 μm, the total thicknessfor the film layer B of 3.0 μm, the thickness per one layer of the filmlayer C of 0.125 μm and the total thickness for the film layer B of 3.0μm.

Then, the same procedures as those in Comparative Example 1 wererepeated to obtain a magnetic recording medium.

The characteristics of the obtained biaxially oriented laminate film andthe magnetic tape are shown in Table 7.

Example 34

The same procedures as those in Example 31 were repeated except forchanging the stretching factor and the discharge rate for each of thelayers, and changing the number of layers as 9 layers for the film layerB and 8 layers for the film layer C with the film layers B beingdisposed on both ends, to obtain a biaxially oriented laminate filmhaving a Young's modulus of 8 GPa in the longitudinal direction, aYoung's modulus of 6.5 GPa in the transverse direction, the thicknessper one layer of the film layer B of 0.533 μm, the total thickness forthe layer B of 4.8 μm, the thickness per one layer of the film layer Cof 0.15 μm, and the total thickness for the film layer C of 1.2 μm.

Then, the same procedures as those in Comparative Example 1 wererepeated to obtain a magnetic recording medium.

The characteristics of the obtained biaxially oriented laminate film andthe magnetic tape are shown in Table 7.

Example 35

The same procedures as those in Example 31 were repeated except forchanging the stretching factor and the discharge rate for each of thelayers, and changing the number of layers as 49 layers for the filmlayer B and 48 layers for the film layer C with the film layers B beingdisposed on both ends, to obtain a biaxially oriented laminate filmhaving a Young's modulus of 8 GPa in the longitudinal direction, aYoung's modulus of 6.5 GPa in the transverse direction, the thicknessper one layer of the film layer B of 0.073 μm, the total thickness forthe film layers B of 3.6 μm, the thickness per one layer for the filmlayer C of 0.050 μm, the total thickness for the film layers C of 2.4μm.

Then, the same procedures as those in Comparative Example 1 wererepeated to obtain a magnetic recording medium.

The characteristics of the obtained biaxially oriented laminate film andthe magnetic tape are shown in Table 7.

Example 36

The same procedures as those in Example 32 were repeated except forchanging the stretching factor and the discharge rate for each of thelayers to obtain a biaxially oriented laminate film having a Young'smodulus of 8 GPa in the longitudinal direction and a Young's modulus of8 GPa in the transverse direction.

Then, the same procedures as those in Comparative Example 1 wererepeated to obtain a magnetic recording medium.

The characteristics of the obtained biaxially oriented laminate film andthe magnetic tape are shown in Table 7.

Example 37

The same procedures as those in Example 32 were repeated except forchanging the stretching factor and the discharge rate for each of thelayers to obtain a biaxially oriented laminate film having a Young'smodulus of 5.5 GPa in the longitudinal direction and a Young's modulusof 12 GPa in the transverse direction.

Then, the same procedures as those in Comparative Example 1 wererepeated to obtain a magnetic recording medium.

The characteristics of the obtained biaxially oriented laminate film andthe magnetic tape are shown in Table 7.

Comparative Example 11

The same procedures as those in Example 31 were repeated except forchanging the inert particles contained in the resin for the film layer Bto 0.02% by weight of silicone particles of 1.2 μm average particlediameter, and 0.4% by weight of silica particles of 0.1 μm averageparticle diameter, and changing the stretching factor and the dischargerate for each of the layers in Example 31 to obtain a biaxially orientedlaminate film having a Young's modulus of 8 GPa in the longitudinaldirection and a Young's modulus of 6.5 GPa in the transverse direction,a thickness per one layer of the film layer B of 0.072 μm, totalthickness for the film layer B of 1.8 μm, a thickness per one layer ofthe film layers C of 0.175 μm, and the total for the thickness of thefilm layer C of 4.2 μm.

Then, the same procedures as those in Comparative Example 1 wererepeated to obtain a magnetic recording medium.

The characteristics of the obtained biaxially oriented laminate film andthe magnetic tape are shown in Table 7.

TABLE 7 Comp. Comp. Example Example Example Example Example ExampleExample Example Example Unit 31 32 33 34 35 36 37 9 11 Layerconstitution Multi- Multi- Multi- Multi- Multi- Multi- Multi- SingleMulti- layered layered layered layered layered layered layered layeredlayered Film thickness 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 Per one layerB μm 0.216 0.168 0.120 0.533 0.073 0.168 0.168 — 0.072 Total for layer Bμm 5.4 4.2 3.0 4.8 3.6 4.2 4.2 6.0 1.8 Per one layer C μm 0.025 0.0750.125 0.150 0.050 0.075 0.075 — 0.175 Total for layers C μm 0.6 1.8 3.01.2 2.4 1.8 1.8 — 4.2 Number of layers Layer B — 25 25 25 9 49 25 25 —25 Layer C — 24 24 24 8 48 24 24 — 24 Total layer — 49 49 49 17 97 49 49— 49 Ratio of polyolefin wt % 8 25 43 16 34 25 25 0 64 Young's modulusFilm forming direction GPa 8.0 8.0 8.0 8.0 8.0 8.0 5.5 8.0 8.0 Widthdirection GPa 6.5 6.5 6.5 6.5 6.5 8.0 12 6.5 6.5 Temperature expansionppm/° C. 7 7 8 7 7 −2 −8 7 8 coefficient Humidity expansion ppm/% RH 119 6 10 7 6 4 12 4 coefficient Track deviation ppm 862 705 548 783 626367 100 940 391 Film forming property — ∘∘ ∘∘ ∘ ∘∘ ∘∘ ∘ ∘∘ ∘∘ x

Example 38

A polyethylene-2,6-naphthalene dicarboxylate resin (PEN) was obtained bythe same method as in Comparative Example 1 except for changing thespherical silica of 0.3 μm average particle diameter as inert particlesto 0.1% by weight. After drying the obtained PEN (intrinsic viscosity:0.62) at 180° C. for 6 hours, it was supplied to an extruder heated to300° C. On the other hand, as the polyolefin (b), a syndiotacticpolystyrene (grade: 130 ZC, manufactured by Idemitsu Chemical Co.) wassupplied to another extruder heated to 280° C. and in a state melted,respectively, the PEN layer B and the syndiotactic polystyrene layer Cwere laminated alternately inside the die to 49 layers being representedas B/C/B---B/C/B, and they were molded in a state being kept in thelaminate structure from a die into a sheet-like shape. Further, anot-stretched film cooled and solidified from the sheet by a coolingdrum at a surface temperature of 60° C. was introduced to a group ofrolls heated to 140° C. and, after stretching by 3.6 times in thelongitudinal direction (MD direction), it was cooled by a group of rollsat 60° C.

Successively, the longitudinally stretched film was introduced into atenter while being held at both ends by clips and stretched by 4.0 timesin the direction perpendicular to the longitudinal direction (transversedirection) in an atmosphere heated to 150° C. as the highest transversestretching temperature. Then, heat setting was applied in the tenter at220° C. for 5 sec and, after being subjected to 1% heat relaxation at200° C., it was gradually cooled uniformly to a room temperature toobtain a biaxially oriented laminate film of 5 μm thickness. The averagethickness for each of the layers was 0.1 μm.

The ratio of the polyolefin (weight %) was determined based on thespecific gravity of 1.36 g/cm³ of the PEN film and the specific gravityof 1.04 g/cm³ of the syndiotactic polystyrene film.

The characteristics of the aromatic polyester resin (a) and thepolyolefin (b) used and the characteristics of the obtained biaxiallyoriented laminated film are shown in Table 8.

Example 39

The same procedures as those in Example 38 were repeated except forchanging the laminate structure from 49 layers to a 5-layered structurerepresented by B/C/B/C/B, to obtain a biaxially oriented laminate filmof 5 μm thickness. The average thickness for each of the layers was 1μm.

The characteristic of the obtained biaxially oriented laminate film isshown in Table 8.

Example 40

The same procedures as those in Example 38 were repeated except forchanging the laminate structure of the PEN layer B and the syndiotacticpolystyrene layer C from 49 layers to 2 layers represented by B/C, toobtain a biaxially oriented film of 5 μm thickness. The averagethickness for each of the layers was 3 μm for the layer B and 2 μm forthe layer C.

The characteristics of the obtained biaxially oriented film are shown inTable 8. While the film of this example satisfied heat resistance andbreakdown voltage, curling occurs and inter-layer peeling was observedfor the film.

Comparative Example 12

The same procedures as those in Example 38 were repeated except forchanging the laminate constitution to one layer of the PEN layer B andnot laminating the syndiotactic polystyrene layer C to obtain abiaxially oriented film of 5 μm thickness.

The characteristic of the obtained biaxially oriented film is shown inTable 8.

Comparative Example 13

The polyethylene-2,6-naphthalene dicarboxylate resin of ComparativeExample 12 was changed to a polyethylene terephthalate resin and, afterdrying at 170° C. for 3 hours, it was supplied to an extruder heated to280° C. and molded from a die at 290° C. into a sheet-like shape.Further, a not-stretched film cooled and solidified from the sheet by acooling drum at a surface temperature of 20° C. was introduced to agroup of rolls heated to 90° C. and, after stretching by 3.6 times inthe longitudinal direction (MD direction), it was cooled by a group ofrolls at 20° C.

Successively, the longitudinally stretched film was introduced whilebeing held at both ends by clips into a tenter and stretched by 4.0times in the direction (transverse direction) perpendicular to thelongitudinal direction in an atmosphere heated to 120° C. as the highesttransverse stretching temperature. Then, heat setting was applied in thetenter at 220° C. for 5 sec and, after being applied with 1% heatrelaxation at 200° C., it was gradually cooled uniformly to a roomtemperature to obtain a biaxially oriented laminate film of 5 μmthickness.

The characteristics of the aromatic polyester resin (a) and thecharacteristics of the obtained biaxially oriented film are shown inTable 8.

TABLE 8 Example Example Example Comparative Comparative Unit 38 39 40Example 12 Example 13 Film thickness μm 5.0 5.0 5.0 5.0 5.0 Polyolefinratio Wt % 43 34 34 — — Aromatic polyester (a) Kind PEN PEN PEN PEN PETMelting point (Tma) ° C. 270 270 260 270 260 Polyolefin(b) Melting point(Tmb) ° C. 270 270 270 — — Dielectric constant 2.6 2.6 2.6 — —Dielectric loss 0.0002 0.0002 0.0002 — — Temperature expansion ppm/° C.7 7 7 7 7 coefficient Humidity expansion ppm/% RH 6 7 7 12 12coefficient Breakdown voltage V/μm 500 480 460 400 450 Heat resistance °C. 120 120 120 120 95 Film curling property — ∘ ∘ x ∘ ∘

1. A single layered or laminated biaxially oriented film comprising athermoplastic resin composition (c) of a polyethylene-2,6-naphthalenedicarboxylate and a syndiotactic styrene polymer having a melting pointof from 230 to 280° C., wherein the ratio of the syndiotactic styrenepolymer is from 5 to 30% by weight based on the entire weight of thefilm, and the film thickness is from 1 to 10 μm, wherein the breakdownvoltage of the biaxially oriented film exceeds 460 V/μm and the heatresistant temperature of the biaxially oriented film is 110° C. orhigher and the syndiotactic styrene polymer in the film comprising thethermoplastic resin composition (c) is dispersed in an island shape. 2.The biaxially oriented film according to claim 1, wherein the biaxiallyoriented film is a single layered film.
 3. The biaxially oriented filmaccording to claim 1, wherein the biaxially oriented film is a laminatedfilm, at least one layer of said laminated film is a film layer Acomprising a thermoplastic resin composition (c) of thepolyethylene-2,6-naphthalene dicarboxylate and the syndiotactic styrenepolymer, and a film layer B comprising the polyethylene-2,6-naphthalenedicarboxylate is laminated to at least one surface of said film layer A.4. The biaxially oriented film according to claim 3, wherein the filmlayer A comprises a thermoplastic resin composition (c′) of from 5 to95% by weight of the polyethylene-2,6-naphthalene dicarboxylate and from5 to 95% by weight of the syndiotactic styrene polymer, and thethickness of the film layer A is from 5 to 95% based on the thickness ofthe laminated film.
 5. The biaxially oriented film according to claim 2or 3, wherein the syndiotactic styrene polymer in the film layercomprising the thermoplastic resin composition (c) has an average lengthin the MD direction of 20 μm or less.
 6. The biaxially oriented filmaccording to claim 5, wherein the thermoplastic resin composition (c)further comprises a thermoplastic amorphous resin (d) having asolubility parameter between the polyethylene-2,6-naphthalenedicarboxylate and the of syndiotactic styrene polymer from 0.1 to 10% byweight based on the thermoplastic resin composition.
 7. The biaxiallyoriented film according to claim 6, wherein the thermoplastic amorphousresin (d) is selected from the group consisting of an acrylic acidcopolymerized polyolefin and a vinyl oxazoline copolymerized polyolefinresin.
 8. The biaxially oriented film according to claim 3, wherein thebiaxially oriented film is a three layered film wherein the film layersB are laminated on both surfaces of the film layer A.
 9. The biaxiallyoriented film according to claim 3, wherein the number of the filmlayer(s) A and the film layer(s) B in total is at least four.
 10. Thebiaxially oriented film according to claim 1, wherein the syndiotacticstyrene polymer has at least one of the characteristic of a dielectricconstant of less than 3.0 and a dielectric loss of less than 0.001. 11.The biaxially oriented film according to claim 1, wherein the humidityexpansion coefficient in the width direction of the film is from0.1×10⁻⁶ to 13×10⁻⁶%/RH %.
 12. The biaxially oriented film according toclaim 1, wherein the temperature expansion coefficient in the widthdirection of the film is from −5×10⁻⁶ to 15×10⁻⁶%/° C.
 13. The biaxiallyoriented film according to claim 1, wherein the Young's modulus both inthe film forming direction and in the width direction of the film is 5GPa or more and the total for both of them is 22 GPa at the greatest.14. The biaxially oriented film according to any one of claims 1 or 11to 13 capable of being used as a base film for a magnetic recordingmedium.
 15. A magnetic recording medium comprising a biaxially orientedfilm according to any one of claims 1 or 11 to 13, and a magnetic layerdisposed on one surface thereof.
 16. The biaxially oriented filmaccording to any one of claim 1 or claim 11 capable of being used as abase film for a film capacitor.
 17. A film capacitor comprising abiaxially oriented film according to any one of claim 1 or claim 11 anda layer D comprising an oxygen atom-containing compound disposed atleast on one surface thereof in which the thickness of the layer D tothe entire thickness of the film and the layer D is 30% or less and the(oxygen atom/carbon atom) ratio at the surface of the layer D measuredby X-ray photoelectron spectroscopy is 10% or more.
 18. A film capacitorcomprising a biaxially oriented film according to any one of claim 1 orclaim 11 and a metal layer disposed at least on one surface thereof. 19.A single layered or laminated biaxially oriented film comprising athermoplastic resin composition (c) of an aromatic polyester (a) and apolyolefin (b) having a melting point of from 230 to 280° C., whereinthe ratio of the polyolefin (b) is from 2 to 60% by weight based on theentire weight of the film; the film thickness is from 1 to 10 μm; thebiaxially oriented film is a single layered film; the polyolefin (b) inthe film layer comprising the thermoplastic resin composition (c) isdispersed in an island shape and the average length thereof in the MDdirection is 20 μm or less; the thermoplastic resin composition (c)further comprises a thermoplastic amorphous resin (d) having asolubility parameter between the aromatic polyester (a) and thepolyolefin (b) of from 0.1 to 10% by weight based on the thermoplasticresin composition; and the thermoplastic amorphous resin (d) is selectedfrom the group consisting of an acrylic acid copolymerized polyolefinand a vinyl oxazoline copolymerized polyolefin resin.
 20. A singlelayered or laminated biaxially oriented film comprising a thermoplasticresin composition (c) of an aromatic polyester (a) and a polyolefin (b)having a melting point of from 230 to 280° C., wherein the ratio of thepolyolefin (b) is from 2 to 60% by weight based on the entire weight ofthe film, the film thickness is from 1 to 10 μm; the biaxially orientedfilm is a laminated film, at least one layer of said laminated film is afilm layer A comprising a thermoplastic resin composition (c) of thearomatic polyester (a) and the polyolefin (b), and a film layer Bcomprising the aromatic polyester (a) is laminated to at least onesurface of said film layer A; the polyolefin (b) in the film layercomprising the thermoplastic resin composition (c) is dispersed in anisland shape and the average length thereof in the MD direction is 20 μmor less; the thermoplastic resin composition (c) further comprises athermoplastic amorphous resin (d) having a solubility parameter betweenthe aromatic polyester (a) and the polyolefin (b) of from 0.1 to 10% byweight based on the thermoplastic resin composition; and thethermoplastic amorphous resin (d) is selected from the group consistingof an acrylic acid copolymerized polyolefin and a vinyl oxazolinecopolymerized polyolefin resin.